A fresh human recruit armed with a shock rifle, rockets, and the morale to carry out the orders of their commander. They are the backbone of the early game for Outpost and are respectable for their decent anti-air capabilities, cheap cost, and expendability.
Marines carry shock rifles for ground-to-ground engagements and rockets for ground-to-air ones respectively. Their ground capabilities are respectable but easily outclassed by other bulkier and harsher units, though their air-seeking rockets are effective at controlling the skies early on.
Marines are the cheapest unit in the game, only costing 2 Manpower each, and even potentially sustainable early on for Communist governments. For this reason, Marine losses hurt less than other units, and they can be quickly deployed to recuperate lost numbers.
The standout weapon for Marines is their anti-air rocket weaponry. Although not much in comparison to proper anti-air units, a crowd of Marines can turn airspace near them into a danger zone for encroaching Scout swarms and the like. If used with proper micro, they can advance on Scouts early on to shoot them out of the air, retreat, then return again to finish off the surviving units.
Marines bear unremarkable ground damage individually, not able to quickly eliminate even A25-Romans without suffering losses. Against Chronite, they are nearly useless, easily blasted apart by Chronite Tanks and whittled away by Gröditz.
Marines compete for being one of the weakest units in terms of health, bearing a measly 62 health that even the base can eliminate in one or two shots. Since they're usually used in groups, splash damage from Chronite Tanks and Missiles are a death sentence, killing them all immediately. Their low health makes them ineffective targets for Meditecs to stand by and heal as well.
Marines are not known for having impressive range. Chronite Tanks outrange them and kill them immediately with one shot, and Snipers outrange them outright and can kill Marines before they even reach retaliation range. Even the Base outranges Marines, killing individual ones before they can even fire their weapon.
Marines are the perfect early-game counter to Forge opponents amassing Scouts, if proper micro is used. Quickly build a sizable marine group, around 10 or so, and push to the middle or first quarter of the field. From there, you can micro your Marines to launch their rockets at incoming Scout swarms, making sure to retreat to avoid taking damage from pulse shots, return to fire more rockets, and repeat until you eliminate the swarms. This takes high focus and understanding of the Marine rocket reload time of 6 seconds.
Being such a cheap unit, it's possible to use singular or small groups of Marines as missile bait to protect your higher value units, if you don't have access to Special armor units that can resist such incoming damage.
Some units benefit greatly from focused fire and only needing to handle one unit at a time. You can use Marines as a cheap way to crowd your front with units to bait enemy targeting away from your higher value units. This can especially be useful for enemies that have weapons effective such higher value units, rather than cheap Marine fodder.