Large scale nuclear war is highly unlikely howewer an attack involving few warheads will become more and more likely as technology proliferates. For example some "nowheristan" aquiring few nuclear warheads and using them to cripple a neighbouring nation it can't take on by conventional means is entirely plausible scenario in next 50 years.
A nuclear nation attacking nuclear nation is unlikely because of fear of retaliation, even crazy politicians running rogue nations are not suicidal, they like to enjoy luxuries their status gives them and that does not mix well with pissing off another nuclear nation or even a nation with highly capable conventional forces and poweful allies.
On the other hand if nations who were previously hostile aquire nuclear weapons it can lead to peace. Remember how Indo - Pakistani conlicts died out after both nations become nuclear powers.