At the risk of sounding patronizing, the usual way to win is to consistently win battles at a profit. You do this by attacking enemy units that are out of position, and avoiding placing units in a deadzone yourself.
Lets start with the clear case where 10 tanks are attacking 2 tanks in a 2IPC territory.
The tanks will almost certainly kill 10 IPC (2 tanks) + 2 territories - 2 (33% chance of losing a tank) for a net 10 profit.
The 2 tanks are in a dead zone and should have been retreated by the other player.
Among good players, often you will see two big opposing stacks and 1-2 infantry on the buffer territories that are traded back and forth. This is because if either player advances their stack, it’s likely in a deadzone (because this extends the attacker’s supply line distance and reduces the defender’s. also fighters can’t land on a territory taken the same turn). So, each player attacks the border territories with just enough fighters and infantry to take the territory with 1 inf left to gain the IPC from the territory. This happens until one stack gets strong enough that it can advance forward safely. IF the advancing stack placed the defending stack in a deadzone, the defending stack must retreat.
In general, it’s better to retreat units instead of leaving 1 infantry to die in a dead zone. However, you also can’t be too passive and retreat your stack to the capital. The optimal positioning usually involves big stacks 1 territory from the traded territories to put maximum influence on the traded territories and prevent the advancing opponent from moving forward safely.