The Terran Infantry are a mixed lot.  You have skirmishers,
rank and file infantry, and covert spellcasters.  Combined,
these forces can take on anything in the game fairly well. 
Each unit has many strengths and weaknesses beyond their obvious
statistics, so we'll be clarifying those for you here. 
Without further ado, let us begin an in-depth review of the Barracks
units for Terran!

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style="font-style: italic;">The only thing better than one
grizzled Marine, is a hundred of them firing at the same target.
 Stimpacked, of course.

Marines

Cost - 50 minerals, 1 supply

Build Time - 25 seconds

Combat Stats - 45 (55 upgraded)  HP, 6 damage with +1 per
attack upgrade, 5 range, 0 armor.  Light, Biological armor
types.

Can shoot land or air units.





Early Game - Unless going against an opponent massing purely armored
units (Stalkers, Marauders, or Roaches), you'll want to have a good
chunk of Marines with your force at any given time.  
After your first 2 mules or so, you should have an abundance of
minerals to spend, and this is generally where they go.  Your
first Barracks should have a Tech Lab add-on so you can adapt early to
your opponent, but your second should be dedicated to pumping out the
trusty Marines.  Since they have a fairly high build time for
being such a frail unit, you can be vulnerable early if your defenses
are only Marines.  Use a bunker or two at your expansion/ramp
in order to secure your expansion from early aggression.

 

Mid Game - Middle game for Marines is about when your first +1 attack
finishes and Stimpacks are done.  Your Marines are killing
machines at this point, but if you're going heavy on the Marines, the
opponent's counter is either ready or in progress.  Send out
SCVs to figure out what your opponent's answer is, and prepare
supporting units to fight it so your Marines can keep being the strong
arm of your force.

 

    Marine Counters and Your Answers

    Banelings - Hellions, run away while
stimpacked until they are picked off.

    Infestors - Ghost EMP, Siege Tanks

    Colossi - Vikings, Siege Tanks

    High Templar - Ghost EMP, Hellions

    Ravens - Ghost EMP, Vikings, even a Thor


    Hellions - Siege Tanks, Marauders with
Slow

    Sentries - Ghost EMP, cry softly as
there's nothing you can do about forcefield.

 

Late Game - Once Medivacs and your opponent's counter units come into
play, you have to be a lot more careful with your Marines. 
You cannot just stand still and fire when all of the area of effect
weapons are on the battlefield.  Micromanagement comes into
play more than ever once the Colossi storm the battlefield, or the High
Templar storm it quite literally.  Marines are quite nimble
when under the effects of Stimpack, and need to be moved away from
these chaotic abilities before they are charred to a crisp. 
Keep upgrading your attack, as Marines are one of the most effective
units in the game to be upgraded for what they do.

 

You might be tempted to use Medievac drop tactics in the Mid and Late
game, but be careful and drop only to harass, not to destroy. 
An intercepted drop with half of your army involved usually spells
instant defeat.  Stimpack and take out what you can--key tech
structures or workers are your highest priorities.  If you can
get away with it, take out your opponent's Nexus/Command
Center/Hatchery to really punish them.  Be careful when moving
Medievacs with your army though--issuing attack move commands or move
commands across long distance will have the Medievac far ahead of your
army.  This can either mean the Medievac is picked off
immediately, or your army is picked off with no healing
available.  Either one is trouble, so move your forces slowly
if you're controlling them all in one attack group.

 

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style="font-style: italic;">Roach Exterminators.

style="font-style: italic;">Harass Stoppers.

The Marauder is here to stay.

Marauders 

Cost - 100 Minerals, 25 Gas, 2 Supply

Build Time - 30 Seconds

Combat Stats - 125 HP , 1 armor, 10 damage and +10 bonus damage vs.
armored (Each +1 per weapon upgrade), 6 range.  Armored and
Biological armor types.  Can be upgraded to slow the units
they attack briefly.

Only attacks ground units and structures.



Early Game - A quick pack of Marauders can force your opponent to tech
switch in a hurry.  Marauders demolish the units they counter,
with good range, outstanding damage, and the slow effect ensuring no
one can escape once focused.  Common rush tactics in
multiplayer almost always involve a pack of Marauders simply because
they're so good against their counters, and when the counters are dead,
they demolish structures as well.  Defenses go down in only a
few volleys from Marauders.  The only thing you have to fear
with these is fear itself--no unit aside from the Marauder has a big
bonus vs. Armored units early on.  Maybe you should fear air
units, or mass Zerglings... but once your Concussive Shells upgrade is
finished, micro your units carefully and keep Zealots slowed and
chasing you around endlessly.  Each one that dies is another
Marauder's worth of minerals advantage to you. 

 

If you are going for these early and not focusing on Marines, go for at
least 2 Barracks with tech labs and get both gas geysers up
ASAP.  You need the extra gas to get your critical upgrades
for these out--+1 attack and armor, Stimpack, and Concussive
shells.  This is 400 gas, and unless you don't plan on teching
at all beyond these, you'll need the 2nd geyser quickly to afford any
of those while producing Marauders full time.  
Strike early and strike hard.  You should try to hide your
Marauder pack to the best of your ability or you'll see Air or
anti-armored out in a hurry.

 

Mid Game - Now that the counters to Armored are in the field, you need
to be a little more careful with these.  Immortals love
Marauders.  Their shields prevent the bonus damage from being
applied, and they kill a Marauder in 3 hits regardless of attack or
armor upgrades.  Mutalisks can have their way with you, and
Hydralisks out-damage you since they don't receive the bonus damage,
being light units.  Siege Tanks can take advantage of your
clustering fire line and demolish you with splash damage while you work
on the forces up front.  Micromanagement again becomes the
name of the game, and you need to focus fire the targets these are
effective against, and let the rest of your army handle the remaining
units.  You don't have to worry about the main counters to
Marines for the most part due to how tough the Marauder is, and how
quickly it can demolish a few of the counters--Infestors and Colossi
namely go down in short order to Stimpacked Marauders.  If you
want to keep giving these a whole lotta love, make sure you go for
Medievacs.  They may have a ton of HP for infantry, but it
wears down over time with each Stimpack.

 

Marauder drop tactics are a little more effective than Marine drops,
but the main problem is having zero answer to air units.  Your
Medievac can simply be assassinated and then your Marauder force is
stranded, without healing.  The good news is that Stimpacked
Marauders can demolish whatever you dropped them off to kill in short
order, and hopefully you can manage to make a somewhat fair exchange
out of the deal, be it a Nexus or several tech structures demolished.

 

Late Game - Marauders are extremely effective against Ultralisks,
Thors, and Colossi.  If your opponent brings any of these onto
the field, a pack of Marauders can bring it down with lightning
speed.  These fare poorly against heavy air, but no worse than
any other unit that can't attack back.  For their cost,
they're still quite tough and since most mid and late game units are
Armored, should remain a staple unit in your force.  Marines
and Marauders comprise the bulk of almost every Terran player's force,
and usually in a 2 or 3 Marines to 1 Marauder ratio.  When
using these, make sure they are leading the pack.  They have
enough durability and range to engage any force, and since they engage
first, if your opponent decides to immediately retreat you'll score
some free kills.

 

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style="font-style: italic;">A rejected costume design for
The Dark Knight, used as concept art for the Reaper.

Reapers

Cost - 50 minerals, 50 gas, 1 supply

Build Time - 40 seconds

Combat Stats - 50 HP, 0 armor, 4 damage per shot, +5 vs. Light (Each +1
per weapon upgrade), fires twice per attack. 
Hurls demolition charges at buildings, dealing 30 damage
each.  5 range, moderately fast.  Can be upgraded to
very fast.

Only attacks ground units and structures

 

Early Game - The only time to effectively use Reapers is right from the
start.  If you're going to get Reapers, build directly to
them.  This means an early Barracks around your 10th SCV,
before your supply depot, and an early Refinery while this is
building.  Start a supply depot once those two are started,
get two more SCVs in the Refinery when it finishes and then immediately
drop the Tech Lab when the Barracks is complete.  Scout with
either the Barracks, or the Supply Depot SCV and check your opponent's
location and see just how vulnerable they will be to the
rush.  If it's a Zerg, watch for a fast expansion. 
If it's a Protoss, if they do more than one Gateway before their
Cybernetics Core, Reapers will do great damage.  Typically you
won't run into Wall-ins in Terran vs. Terran, but if you do, see if
their Barracks' Tech Lab is already complete like yours.  If
it is, you might expect the same rush coming to you, or an early
Marauder, both of which should make you reconsider this attack.

 

Once you have two Reapers, move out.  If you are training a
third, set the rally point to right outside your opponent's base or
right up their cliffside.  Skirt around the outside and make
your way straight for the mineral line.  Unleash hell--it only
takes 3 double-shots of the Reaper to take out a worker.  Do
as much damage as possible, and if the response is Zerglings or
Zealots, just run away from them and take them out slowly.  So
long as you're not on the creep, you can do this to
Zerglings.  The moment a Queen or Roach comes out, your attack
is going to fall apart quickly and you should retreat with your Reapers
intact.

 

At this point you can switch to an offensive contain.  The
Zerg probably is thinking about, or already has put down their Hatchery
at their expansion.  Have your scouting SCV throw down a
bunker next to the expansion (but hopefully in range to pick off units
that leave his base as well) and put your Reapers inside if he's
building the Hatchery.  Reapers are capable of lobbing
explosives right out of the ports of the Bunker, and you will force the
Hatchery to be cancelled or destroyed more than likely.  Keep
your SCV close at hand as the Zerg will likely try and break this in
short order, and keeping the Bunker repaired will thwart an early
breakout no problem.  Once an initial attempt fails, expect
several Roaches for the 2nd effort and prepare accordingly. 
Your tech lab can now produce straight Marauders and rally to behind
the Bunker.  Enjoy your victory, more than likely!

 

With a Protoss opponent, the Reaper Rush can last until a Stalker is in
the field.  Micromanage carefully and gun down as many workers
as possible, and if the opportunity presents itself, take out a chasing
Zealot with some hit and run tactics.  Never have your Reapers
engage in a fair fight!  These units are frail, fragile, and
futile to use as combat units in a shoot-out.  When a Stalker
is out, you should probably retreat and cut your losses.  Put
your Reapers at high ground around your opponent's base and wait for an
opportunity to strike again.  You'll likely only have one or
two real opportunities to strike beyond the initial attack, and even
then they are quickly thwarted by Warp Gates bringing in Stalker
reinforcements.  The same thing applies to a Terran
opponent--don't bother once a Marauder is out.  Once that slow
finishes researching, your Reapers are fodder.  Save them for
scouting and another attempt later on.

 

Mid-Late Game - Don't bother with these.  You just don't get
enough bang for your buck, and expansion raiding is cheaper and more
efficient with Medievac drops of the above combat infantry since they
can use Stimpacks to chase down and kill workers just as fast.

 

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style="font-style: italic;">No longer as specialized as
before, Ghosts are now a threat in any matchup, but are especially
menacing versus Protoss.

Ghosts

Cost - 150 Minerals, 150 Gas, 2 Supply

Build Time - 45 Seconds

Combat Stats - 100 HP, 0 armor, 10 damage (+10 vs. Light) per shot with
+1 to each per attack upgrade, 6 range.  Light, Biological,
and Psionic armor types.

Can attack air or ground units.  Can attack while Cloaked.

 

Early Game - Getting a Ghost early is generally more trouble than it's
worth unless you are facing an aggressive Protoss player.  The
Ghost Academy itself is a hefty 150 minerals and 50 gas, and then you
have to train a Ghost.  This means it costs just as much as
Thor to get the requirements for a Ghost and the Ghost itself
out.  You have to make your first Ghost count, and that all
rests in your ability to EMP.  EMP is the great equalizer of
the Ghost, with a long range and a fairly good radius, though nerfed
since its inception.  EMP destroys all energy to units in the
area, and 100 shields.  This effectively removes the shields
from the entire Protoss Army almost, and most notably the Immortal is
rendered shieldless by this attack, removing its ability to reduce
damage to fractions of what it was.  Many early pushes center
around the Immortal and its strong performance against your Marauders,
and removing the shields from them gives your Marauders much more equal
footing--though still not a fair fight!

 

Mid/Late Game - The Protoss caster-type units are the biggest threat to
the Terran infantry mob by far, and landing an EMP turns them into
harmless civilians or puny 6 damage turrets.  Without this
disable, your army will be split apart, have its damage cut by a third,
and torn asunder by Psionic Storms and Feedback.  Keeping a
Ghost or two with your army and firing off those EMPs at critical
moments will make or break your fights against a smart Protoss player
that uses Sentries--and they all do at this point.

 

Researching Cloak is generally unnecessary.  Ghosts are tough
enough and have enough range to not be picked off very easily, and
after they unleash their EMP, their job is more or less
complete.  If you miss some Templar with the EMP, you might be
in a quick-draw showdown of micromanagement to see which player can
instant kill the other faster--if you can Snipe the Templar in time, or
if they can Feedback you in time.  Feedback sports an
impressive range of 9, so the Protoss player will have the advantage
unless they have their Sentries selected first.  Typically
Protoss players Forcefield first, Psi Storm second, so you have that
period of vulnerability.  All it takes is one Snipe. 
One shot, one kill.

 

The only time I'd go for Cloak personally is when going for a
Nuke.  Nukes are cheaper at 100 minerals and 100 gas each, and
no longer require a worthless add-on to the Command Center or a silly
amount of population.  They take seemingly forever to fall
though, and do less damage than previously to main structures--the
damage cap is now 500 damage per nuke.  You'll need to drop 3
in order to instagib an expansion.  Nukes do have their uses,
and against Zerg especially you can get things done with Nukes that are
unexpected.  Zerg players tend to cluster their tech and unit
structures, and a pair of Nukes will render them unable to make
anything beyond Drones and Overlords once more.  Again, the
downside to this is that you spent 300 gas on 2 ghosts, 100 gas on
cloaking, and 200 gas on the nukes themselves.  That's a 600
gas advantage to the Zerg, and that's 6 Mutalisks, 12 Hydralisks, or a
frightening amount of Roaches or Banelings.  This strike will
likely trigger an instant all-in attack from the Zerg, and you need to
be ready for that!

 

Note that EMP removes stealth from those units hit.  If you're
quick, you can reveal Dark Templar and kill them before they can reach
you, and pick off Observers as they try to stay with your army or scout
your base.