Had a scenario where a player played a Warfare Rider card voting For an agenda card which, on a For vote, would force all players to reduce their fleet down to 2 dreadnaughts and 4 cruisers. The warfare rider card allows the player to put a dreadnaught from their reinforcements into play.

Which of these effects resolves first?

I think it would help if you put the exact wording of the card here.

pg 7 of the Rules Reference

Outcomes 8.22 sub-dot 2

After resolving the outcome of the agenda, resolve any abilities that were dependent upon the predicting outcome

So it sounds like the vote happens, everyone destroys down to their 2 Dreads & 4 Cruisers, and then the player gets to add a Dread