Ideology is a good story to tell the masses, I suppose, but war is ultimately about wealth and resources. And living forever is going to make resources scarce in ways we really can't imagine from our current perspective. In my lifetime the population of the planet has doubled (and the population of your country, Bhupendra, has very nearly tripled in that same time) -- and that's just because of an overall decrease in infant mortality and a small increase in global average life expectancy. The projection is that we'll be at 10 billion or so in another forty years, and that really doesn't take into account the possibilities that radical advances in medicine might well tack twenty or more years onto the average life expectancy.
If we all live forever, if death is something that only occurs rarely, and then only from massive trauma, we're going to have to put an end to this "having children" habit some people have. Conserve as we might, sooner or later there isn't going to be enough rice and beans to feed everybody (meat will have become a super-expensive luxury, and the oceans will have been fished clean). And that will all happen years before we are able to terraform and migrate substantial numbers to another planet.
Hunger is a powerful motivator. Crime will increase first, and food theft will go from being a forgivable offence to the moral equivalent of running a major drug ring. (And the Javerts of this world will not throw themselves into the Seine to avoid arresting the Valjeans.) Eventually, tribalism will rear its ugly head -- the reason we're starving is because they can't keep their population under control! War is inevitable.
And the worst part of all is that we won't learn a thing from it. The scale of war that will occur will kill off billions -- enough to temporarily stay the crisis. Oh, we'll build memorials and sit through moving and passionate films about the horrors of war and how it must never happen again, but that is the least we can do -- and therefore the most that we will do. Will we look to the heavens and plan ahead? Not a chance. No, we'll complain about taxes and the waste of resources being funneled into a pointless space program after sending another dozen robots and a token astronaut or two to Mars, then sit back, watch whatever passes for "television entertainment" in that future, and slide obliviously to yet another food crisis.
