Does anyone have a solution for this mission, the enemy appears to have overwhelming resources? Gaining the other country doesn't give you enough firepower.

Nothing overwhelming there,really.You just have to conquer neutral as fast as possible and disclose your secret ally.(in my case I bought there 2 turrets)

I just finished that mission today. The way I did it was like this: My main force in Rogeria backed downwards to buy me some time, and to prevent that enemy cannon from firing on them. I sent the southern rocket launcher up to my main force (and left the neutral country alone), positioned my tanks and the smallest units between my two launchers and the enemy force (my launchers were just below the border to Rogeria) and the heavy bots on that mountain to the west.

At the first possible chance, I disclosed my secret ally and bought two launchers (one in the city and one in the hex to the east of the city). The first one got killed the first turn (that was planned) by the enemy rocket launcher, but the other one killed the rocket launcher. I then waited a turn and bought another rocket launcher, and then a tank to defend them. This kept a major part of the enemy reinforcements occupied (and was a pretty efficient death trap ), so my main force managed to survive the attack. Eventually, the enemy spent all his money. The rest (attacking the neutral country and the two enemies) was easy, as he didn't have anything other than the guerilla forces to defend himself with.

If I remember right, this was the one with 2 enemies, one to the north and one to the south? If so, then the way I beat it was to concentrate all my forces on the southern enemy. I invaded, overwhelmed him, then got the indemnity. After getting that extra $8 boost, I then focused all my forces back north and stormed over the enemy there.