I think it really depends.
We're pretty developed, yet it doesn't look like we'll be giving up war any time soon unfortunately... and, considering the fact that the levels of war haven't really diminished as humans have developed (infact, with more powerful weaponry, its increased), I think its safe to assume we'll be a war-like species for many years to come, despite further development...
If said aliens are similar to us (as in, humanoid, similar thought process etc), then chances are, they will be similar to us and will be war like... though they may initially come in peace, political unrest may result in war.
That said, we also have to think of the possibility of aliens being completely different from us - its quite possible that aliens will never have even thought of the concept of war.