They make a land grab every 10 years on average. Diplomatically asking someone not to invade someone else usually doesn't work. Sometimes it does though.

Going to war over a tile blocker spawns 1 post-atomic army (or assault army if you have that tech) per hostile tile blocker, plus 1 rebel army that represent the people living in the tile blocker you're fighting over. So worst case 6 armies max if you clear the first, 8 if you fight them over their first land grab.

90 days later on average they get reinforcements from each TB, 1/3 post atomic (even if you have assault army tech), 1/3rd rebel, 1/3rd no army. So that 8 they started with should get an average of 5 more, of lower quality.

You get the option of calling up you own reinforcements, if you can afford them. You get a 50/50 chance of an army from each of your pops, either post-atomic or rebel as you choose. So an average of at least 5 instant armies, as long as you can afford to equip them. Your worst case reinforcements are equal in number and can be superior in quality to their best case reinforcements, as long as you save up enough beforehand to fund your war.

Or you can bust out a nuke and insta kill 5 of their armies. They will most likely kill 5 of yours right back, with almost as good odds that they kill one of your builtup tiles instead. there's a chance they roll over and take it, and there's a chance it goes all out and 1/4 of your pops end up in peace on a tomb world with no one left to fight them/you anymore and plenty of food. Build a couple fallout shelters and you'll get the habitability back.

If the option text doesn't make sense, the mouseover tooltips may help a bit. Not that much, but a bit.

I'll welcome serious feedback about adjusting the timing of reinforcements, how many armies should come from friendly and enemy tiles on average, the power of nukes, and the odds of the various nuke responses.