The first US troops are arriving in Israel today, part of a build-up for what officials are calling the single largest joint US-Israeli military exercise in history, a drill that will last three weeks and include 3,500 US troops in Israel and Europe.

Officials say the drill aims at preparing a multi-front defense against mortar fire, short and long range missiles. The drill was initially planned for earlier this year, but was postponed because of “regional instability.”

At the time the drill was being couched as preparing for a possible Iranian retaliation to an Israeli attack, and there was a belief that the postponement was meant to show US opposition to Israel’s initial ambition to start the war ahead of the US presidential election.

It isn’t clear, however, how “regional instability” is any less of a factor now than it was in January, and if anything regional tensions seem to be worsening as neighboring Syria implodes in an ugly civil war. Still, with military drills more a diplomatic than a strategic matter, the timing may not be so significant.