Often it's in the interest of powers to cooperate. It would be nice if there was a mechanic for that.
Good news, there is! The players supporting those Powers simply need to choose not to Undermine each other this week - and the game provides plenty of incentive for them to follow that strategy.
- Undermining is generally less efficient activities intrinsically than the equivalent Reinforcement activity (also often slightly more difficult and much worse paying)
- Undermining is often given an extra penalty via System Strength Penalty "just because"
- most weekly tasks are Reinforcement or Acquisition rather than Undermining
- if you want a new system, you have to pay at least the same Acquisition costs whether it was one you'd previously undermined or one which has always been neutral, but the undermining itself will cost something - and probably considerably more, if it and/or the subsequent reacquistion is opposed.
As a result, the ZYAJDANELPAF alliance is regularly adding hundreds of systems a week to its cooperative project to control the entire bubble, with around 75 million control points of net Reinforcement/Acquisition over Undermining being added every week.
Some people say that it would be nice to have a mechanism to encourage Powers to fight each other, but I don't think we need one of those. That 75 million might sound big, but Colonisation will soon start expanding the bubble even faster than that, so wasting time attacking each other is just a bad idea when it comes to the ultimate goal of turning every system into a max-strength Stronghold.