Ok, so if both of these hypothetical parties fully and completely, without reservation, support the invasion (or let's call it what it is, war) then there is not a lot you can do.
But it turns out one party has no problems with that invasion/war and supports it. Heck, they even tell others who protest the invasion to just accept it all as a fact and just deal and live with it. Also calls to surrender parts of your land because the leader of the invasion is having a certain 'grip' on the party leader.
The other party is lacking in action to change this outcome but has shown signs that they do not like the invasion but need to get/stay in power to try and stop the madness. But to get elected they cannot come out and say "we will stop the invasion" because that is a death-knell to their goal to be elected and in power.
As you see or can probably understand, you are not dealing with parties per-se, you are dealing with the populace who get to vote for either party. Navigating that populace to get elected is a tricky and a risky thing, before you know it you blew you chances and the other party wins.
This is truly the case of voting for the party who has some semblance of being able to do the right thing, even if it is late or voting for the party who has clearly signaled to be 100% against what you hope and stand for.
Best bet in this case is to vote for hope and possibility, not the surefire way the other party wants to dig a much, much deeper hole which will be infinitely more difficult to crawl out of.
There is no easy solution, only thing we have is choosing the ones who show a flicker of hope in doing better. Good luck!
This might be the case, this is the 'hope' part I mentioned but it might work.
But you can bet on your life and those you care for that the other party will try everything in their new found power to make things absolutely worse.
I get the dilemma and voting for any party in this scenario is tough but I would still opt for the party of hope and which displays signs of change for the better.