Well, eventually combat will get So quick it'll be ending up as whoever fires first wins the war.
I doubt this, if only because of second-strike capabilitiy. Plus all combat will need to be thought out in advance, so it will take a long time to get ready.
The idea of relativistic weaponry is that there's no second-strike capability left.
Edit: A bit late, but nukes don't cause an EMP in space.
I said the latter.
Also, If you guys are under the impresssion second-strike capability can be destroyed, your wrong. Space is a BIG place. Right now, even at our current technology, the only way to wipe out the united states ability to strike back would be burning the oceans and stripping the atmosphere. Even a small, rouge planet outside a galaxy could hold something, and if it does, that's all you need to strike back, and hard. In fact, you could combine this with Relatavistic weaponry and go even further, saying whenever a nuclear attack lands, the attacked government will use second-strike capability and wipe the opposing enemy from existance.
The logical conclusion then is something I thought up a while ago, 1/2 strike capability. Basically, in a future where 2 nuclear powers, or whatever weapons, have time altering or controling technology, at the start of a war a hidden nuke or nukes are sent into the past to get the jump on them. Obviously, either the opponent beats them to the punch, or they are injured or destroyed, triggering the nuclear war in that timeline. At that point, the other side will send a nuke back, and push the start of the war back further. This continues until we reach the beggining of time-travel technology, where, unless the governments have made previous arrangements, the war will finally end, or, begin, per se. At this point, both civilizations will be hevaily destroyed, or only one will. Regardless, therefore the moment a government invents the system, if it is ever used, a nuclear war will imediately commence.
Also, considering we currently have no way of FTL, there is nothing to say it can't be predicted by unknown methods, or leave a signal.
TL;DR, The basics of Second strike capability and time-travel are more then enough detterent. This isn't anything new. Its the Cold War all over again. With all the same problems, but with fancier machinery. And more damage. Also, this is what happens when you argue with a nuclear-detettence fanatic. I know alot on teh subject.