This is beyond a personal preference.
This is game player - game play... each has their own and some actually (simply) play this way.
It is hard to risk any ally status with a player out side of your alliance - yes, even coalitions can be this way or even worst "common relationships between multiple coalitions" ... etc....
There are a lot of deceit plays to be and will be seen - some in the worst manor and some, simply play.
How you act or react and choose to handle such game play is really based on you as a player.
Don't get me wrong - i personally disagree with breaking a "gentlemen's handshake" and would not - but that does not mean the other truly does honor the same value - game or not.
So:
rebuild - Yes
refocus - Yes
revenge - .... well - idk... depends on the map / time and many factors... sometime - their own dacite kills them off long before it is worth your energy.
My advice - is:
Stay true to yourself as a player.
Stay focused on how you actually win the game - VP's.
Stay smart and humble - do not let one other player - play on emotional or reactive responses from you.
Then given the chance... yeah - smash and take it all from them... lol... JK
but take the lesson as it is - because it will happen and remember the world/ map/ game is small.
Be a honest player first foremost and do what you feel/ think is best.