The idea that it was the Timelords and the first shot fired was what we saw in 'Genesis of the Daleks' makes the most sense to me.
Sure, it wasn't unprovoked as the Timelords saw a time when the Daleks would dominate the universe, but their's was the first use of time travel as a weapon against the other. When it failed, it set the Daleks on a path that would eventually lead them to seek out offensive time travel technology of their own (encouraged along the way by somewhat out of order encounters with The Doctor.)
They didn't mean to start a war, they meant to shoot "the devil" in the back...and missed. What happens after you do that is never going to be good.
So yeah, the Daleks started a war by the very fact of their existence, but the Timelords escalated it to a *time* war(s).
Sure, it wasn't unprovoked as the Timelords saw a time when the Daleks would dominate the universe, but their's was the first use of time travel as a weapon against the other. When it failed, it set the Daleks on a path that would eventually lead them to seek out offensive time travel technology of their own (encouraged along the way by somewhat out of order encounters with The Doctor.)
They didn't mean to start a war, they meant to shoot "the devil" in the back...and missed. What happens after you do that is never going to be good.
So yeah, the Daleks started a war by the very fact of their existence, but the Timelords escalated it to a *time* war(s).