You have to use the retreat button on the bottom right (or hit double Q as a hotkey). This will make the units retreat automatically. They dont immediately disappear and can still get killed, but if at least 1 vehicle of the platoon survives the veterancy is kept and you get a slightly shorter "recharge" on the platoon before you can deploy it again (compared to when it is completely wiped).

Originally posted by MagneticJerk:

You have to use the retreat button on the bottom right (or hit double Q as a hotkey). This will make the units retreat automatically.

This is also useful in skirmishes, because you don't have access to supply trucks, so retreating them off board is the main way in skirmish to resupply and repair them. You can also withdraw to the squares you spawn from - but that only resupplies and repairs, it does not replace any destroyed units. Resupply also replaces any infantry killed, as well as resupplying the infantry.

In campaigns, you can send out supply trucks, but withdraw is the only way to get back any lost units. However, in campaigns, you have a limited number of reserves for each unit - you loose all four your four tanks in a platoon, but you've only got a reserve of 2 - you'll only be able to bring that platoon out again with 2 tanks. Loose those, you can no longer field that platoon for the battle.

Oh, and withdrawing your units before they're destroyed is recommended because you keep any experience that unit gained. This is more important in campaigns, as veterancy carries over from phase to phase, but it's still useful if you're playing skirmish.

Another thing about retreating platoons ; when a platoon is totally wiped out, it takes longer for it to be available again.