Infantry units are the unsung heroes of the battlefield: although most players will look to the flashier tanks and choppers to forward their aims, a smart, subtle player can use infantry to harry their foes and destroy much more powerful enemy forces. Infantry units are unique in that they can garrison themselves in buildings to fire on their enemies from within, as well as hide inside forests to prevent enemies from spotting them.
When controlling infantry, keep in mind that they're fairly vulnerable to enemy fire: enemy helicopters will chaingun them to death, while enemy tanks will use offensive abilities or simply run them over. If your troops are out in the open, well…they shouldn't be. You may have to move across open stretches of land, but if you do, constantly jam on your Sprint button to force your troops to move more rapidly. You want to get them to cover or have them moving towards cover at all times. Helpful buttons to remember here are Y and H. Tapping Y will tell all of your targeted infantry to move towards the nearest transport, while hitting H will move them towards any nearby buildings and plant them inside. Pretty handy for when you come under fire.
As multiplayer games creep onward, it'll be much more likely that the control points will have been decimated by tactical aid and that there won't be any buildings or forests for you to hide in. If you find this to be the case, then you may want to consider switching to armor in the late game to help keep your forces alive long enough to capture the control points.
Using Infantry
You have four primary infantry units to use here.
Infantry Squad
Generic infantry is something of a jack of all trades unit, in that they have anti-aircraft, anti-tank, and machinegun capabilities. They won't be overwhelmingly powerful against any particular type of unit, but they can be handy at forestalling enemy advances, especially if you station them inside buildings near control points. Their jack-of-all-trades nature makes them useful, especially since they're the only infantry unit that can fire on enemy helicopters.
Anti-Tank Squad
One of the secret dreads of the armored commander is the suspicion that the trees or buildings that he or she is approaching are filled with anti-tank infantry units. When controlled well, a group of anti-tank soldiers can decimate an entire opposing force of tanks.
Anti-tank soldiers wield rockets that will quickly kill any
armored unit that they fire on. If you can control your units directly
(i.e. if they're not parked in a building), group all of your anti-tank
infantry units together and control their fire, focusing first on
medium tanks (their offensive ability can quickly wipe you out), then
APCs, then moving on to the light and heavy tanks. Of course, if you're
totally exposed, enemy armor will try to run you down, so you may want
to activate your sprint ability and stay moving while you run for
cover.
Anti-tank squads do pack some light machineguns for defending themselves against enemy infantry, but will still fall to infantry squads or snipers before they manage to deal much damage.
Sniper
Snipers require a fair amount of resources, but you only get a single unit here, so it's sometimes tough to justify the expense, especially since snipers can only fire on enemy infantry. That said, they do a bang-up job of eliminating infantry, and can be handy as spotters, but they'll be dead weight unless you come across enemy soldiers, and it can sometimes be a bit of time before that happens in an armor-happy online game. You can always send them out on their own and have them sneak through the forests near enemy drop zones and hope that they get lucky, but all it takes is one napalm strike and they're done for.
Demolition Engineer
Demo engineers are the most specialized of the four infantry types, and are going to be at their best when used to destroy bridges that lead to control points you already own and want to protect. Not all maps will have bridges, but if you do manage to find a control point that needs protection, capturing it with infantry and destroying the access roads with demolitions can make it pretty difficult for armor to get to you without making some major detours. Bridges can be replaced with tactical aid, though, so you won't be able to count on this making a permanent difference in the battle. Sometimes, however, all you need is a temporary diversion for a bit, and blowing a bridge is a great way to get that.
Demo teams can also destroy buildings, which is somewhat less useful than blowing bridges. Generally if you want to blow up a building, it's because there are enemy forces inside, but said forces will often chop through your demo teams before they can reach the building, making this a dangerous tactic unless you're willing to suicide your demo unit. If you do want to destroy a building, be sure to use your sprint ability to get your unit to the target quicker than they would be capable of otherwise.
Troop Transport
The troop transports are lightly armored vehicles that can carry a squad of infantry to the front while also serving some other functions. It has machineguns, so it can fire on enemy infantry squads (although it will quickly be destroyed by any number of default infantry or anti-tank infantry squads), but more importantly, it moves quickly and has a decent range of sight, making it a cheap way to scout enemy positions. It can also slowly perform repairs on other vehicles, so if you have a squad of these, they can all repair each other; this will be done much more slowly than a real repair tank can do, though.
Transport Truck
Unarmed and barely armored, the troop transport's only job is to get infantry to the front. Apart from that, it can't do much, but if you need to capture a control point that's relatively clear of enemy presence, you can leave them on the perimeter points to help you capture and fortify them. Transport trucks will die under the lightest of enemy fire, so they're only useful if your drop zone is very far away from the control points you want to take; even then, you need to carefully monitor them and pop your troops out if they come under fire, as a troop transport will also kill any troops inside of it when it's destroyed.
Defeating Infantry
Enemy infantry poses some problems for your commanders, as soldiers are pretty good about hiding away from your fire. If they head into buildings, they can't be attacked directly, and they're often almost impossible to see if they enter forests.
First off, though, if you encounter infantry in the open, you can try to wear them down with helicopter machineguns. This will take a bit of time, however, since your targets are small. Tanks fare better against infantry most of the time, since you can simply charge forward to a point on the far side and squash most of the infantry under your tracks. Yeah, they'll get some rockets in on you, but you'll rout most of their units if they're bunched up together in a single group (as they often will be). You can also try to use artillery on them, but be sure to lead them a bit, as infantry in the open will almost always be on the move.
More often than not, though, you're going to be facing infantry that's hidden away in buildings surrounding a control point. Garrisoned infantry are difficult to take out with anything other than artillery, since they can fire out of the building onto incoming forces. In order to kill these guys, you'll need to demolish the building that they're in. Tanks or choppers can do this just as well as artillery, but it'll take them longer; use special attacks on the buildings to wear them down more quickly. Unfortunately, a good infantry commander will be ready to unload his forces when the building is about to blow and will usually park them in another one nearby. Thus, your best bet for dealing with pesky infantry, especially when there are multiple units in a single large building, is to use laser guided bombs to instantly blow it away. Your enemies will take time to recover, and you'll also prevent them from using the building in the future.
Infantry in the forests are less protected from gunfire, but are also much more difficult to spot. Helicopters (including attack helicopters) can usually spot infantry in the woods if they get close enough to see them, but usually you'll be alerted to their presence by the smoke trails that lead back to their rocket launchers as they blow up your tanks. Most kinds of tactical aid will smoke them out, but napalm is custom-made for this purpose: lay it down along the treeline, and you'll burn up any infantry in the woods while also destroying the trees and preventing them from taking cover there again. Daisy cutters are another step up the ladder, if you have the aid points; they'll kill anything anywhere near the area of effect, and also destroy the trees as well.