Infantry is undefeatable in forests and towns. Entering those forests requires securing the forest edge, which is best done by fire support – transports and tanks, the cheaper the better.
Forests are usually surrounded by large amounts of open space. Of the ground units, heavy tanks are best at winning battles in the open. ATGM units also work well if you can protect the launch vehicle with distance, screening units or good timing. Autocannons, light tanks and mediums are cheaper options for securing open space – not as good as heavy tanks, but appropriate if you do not want to spend 300 points on a particular front. They all lose heavily to expensive tanks, even when massed.
Recon units sidestep the firepower race of tanks entirely. Due to their stealth, they can prevent enemy infantry from walking into your forest even if the enemy has fire support advantage. To kill a recon lav-25 shooting your advancing infantry while itself remaining invisible, you need to spot it, with recon of your own.
ATGM helos also sidestep the firepower race of tanks. They are poor offensively, but an enemy helo hovering over a forest you intend to attack will prevent your tanks from moving up and securing the forest edge. To drive it off, expensive long-range antihelo AA is needed.
Rocket helos and gunships serve as a unique type of fire support – just like transports they can cover your advancing infantry and fire back on anything that threatens it, but they are not vulnerable to enemy tanks.
Planes are often expensive for the firepower they bring. A 120pt air superiority fighter has a slightly smaller kill chance than 120 points of anti-plane AA. Their main draw is their ability to strike anywhere on the map and their high alpha damage. A bomber can provide instant support to any weak flank that is hit by a surprise attack. Antitank planes help defend your open spaces in a similar way to ATGM helos, but they kill much faster and are countered by heavy SAMs and ASF, not antihelo AA. It may seem like the hardest counter to planes is ground AA, but it is actually fighters, as they can drive off enemy bombers before the payload drops.
Although heavy tanks dominate light tanks in longrange battles over open terrain, they are also most heavily countered by AT planes – a player with three cheap leopards is far more likely to have at least one tank survive a Mig-27 run than a player with a single Leclerc.
That’s it. Infantry is the king of wargame once they get into position in a forest. Tanks secure open space, allowing your infantry entry into the forests and towns that are key to victory. However recon, helos, planes and to an extent ATGM carriers each sidestep the open space dominance of tanks in some way, necessitating additional counters – spotters, AA, ASF, smoke. Hence, combined arms – all focused into allowing or preventing the flow of infantry into key strongpoints.