The Federation and the Klingons were doing pretty well together at the end of Deep Space 9 and we haven't seen much since that would indicated any conflict. However, when Star Trek Online begins, it seems as if Federation-Klingon relations will be back at the state of The Original Series.
If so, how did it happen?
There's a great deal of speculation going on in the forums right now on a thread by Polaron.
Here's my speculation, as posted on the thread:
I think the most likely cause is that the Federation refused to share the technology Janeway brought back from the future. Either they didn’t want to share this new tech with the Klingons because they wanted it for themselves (something very possible considering some of the Federation’s politicians we’ve seen lately) or because they were afraid that distributing the technology would contaminate the timeline.
The Klingons likely took serious disagreement to this, accusing the Federation of trying to keep the Empire on a technologically or political lower level (a not uncommon argument from the Klingons) and threatened the Federation with war if they didn’t give up the tech.
I’d bet the Federation then made transwarp public, in order to try and keep the Klingons happy. However, of course, what the Klingons wanted was the combat-centric tech. In that situation, I could easily see the pre-existing allies of the Federation turn on them, including Worf.