I believe either less resources or religion would be the main cause. I'm not ruling out the escalation of a small conflict, but nations seem to be more all talk nowadays, so I have my doubts there's going to be an escalation of a minor war.
I think resources could be a cause since fossil fuels are running out, and nobody seems to care about finding alternate fuel to sustain us. Religion could also be a cause, since, let's be honest here, religion is just out of control now with terrorists and hate groups, and I hate to say it, but it's probably going to have some really bad long term effects.
"There is no such thing as coincidence, only inevitability" - xxxHoLiC
"Justice? But I don't serve justice, Watson, I serve the truth." - Sherlock Holmes