During the negotiations with the Cardassians on the Enterprise Riker loudly exclaimes “the Federation will not start a war.” Oh please, the Federation have started almost every war we know they’ve had. Especially the Dominion War.
Burnham started the Klingon war and Starfleet started the Dominion war.
Focusing on the Doninion War, Starfleet discovers a stable wormhole to the Gamma Quadrant and starts exploring it. Fine, fair enough.
However in 2371 the Dominion warns Starfleet to stay on their side of the galaxy, stop planting colonies and stop violating Dominion territory. Again, fair enough.
After all would the Fedetation allow the Romulans to explore Federation space freely? Of course they wouldn’t. But the Federation expects to explore the territory of others freely because Starfleet believes they have a RIGHT to explore the galaxy and if that means crossing borders then so be it.
The Federation then gets all pissy when the Dominion sees them as such a threat they ally themselves with Cardassians to establish a permanent presence in the Alpha Quadrant. All this happens outside of Federation space so they have no rights to interfere as per the Prime Directive.
But do they leave the Cardassians to run their civilisation as they see fit? Of course not. They mine the wormhole which starts the war.
So as far as the Dominion War goes, the Federation were the clear aggressors. And let’s not get into the attempted genocide of the Founders.
Spoilers from here on out! I don’t think tags work universally, but I’ll try my best.
spoiler
It was actually the founders infiltrating the Klingon Empire, getting them to act aggressively against the Cardassians to sow dissension amongst the Alpha Quadrant entities. As far as we know the coup against the Central Command was legitimate. We see lots of rumblings about resistance movements against Central Command for a while before this.
Spoilers - Season 3+ DS9
Ah, for some reason I was thinking they had infiltrators in all of the major Alpha Quadrant powers by that point. I must have been mixing it in with what the Romulan changeling said. I really should reference Memory Alpha at times rather than relying entirely on my memory!
Same
It’s entirely possible they do, it was just never explicitly shown. We just know the Klingons use that as an excuse to annex Cardassian territory, and we know (from season four finale/five start) that the Klingon leadership has been infiltrated by changelings. We also know that the new civilian government leaders are blood-tested by Starfleet when they’re saved from Cardassia Prime.