But as the F.N.U. it isn't easy to take back. First, you can't take a tank frontline in case you're facing an PL frontline, so you take LAVs and Mortars... but then you're slow and struggle to react to the PL moves.
The PL advantage is apparent, but as has been pointed out in other threads, SA placement can often be a factor that can put you at a worse disadvantage than moving second.
But as that is not the issue here, I'll refrain from that argument and answer the issue at hand. How do you contain the damage of the PL's first move?
First off, you must take great care in choosing which allies to disclose. The main goal is to have as small as borders as possible so that you can cover them better. Generally this means choosing neighboring allies if at all possible. In this case you know you don't need to cover certain borders.
Once you've chosen your initial allies, cover your borders with LAVs. Don't partially cover them unless you have terrain that inhibits transports, as even a small hole can destroy you if the enemy can sneak a transport past in the first turn. If I have a bad SA layout, I sometimes purchase all LAVs, attempting to cover as many borders as possible.
Once your borders are covered, if you still have cash, choose a front you might like to attack, then add appropriate units to support this front.
If you have your borders covered, and the PL discloses transports and tanks next door, you actually have the advantage! His tanks can do 2 damage a piece, but in the end a bunch of LAVs with mortar backups can handle tanks with ease, inflicting a lot more monetary damage than receiving.
Sure, by playing defensively like this, you may feel like you are not in control, but try it out. You may find that you have less instances where you completely lose control in the first round. If the PL can't invade your territories immediately, then the advantage is over, and the FNU and PL are on equal footing.