Comment by phantasmish

They could do that now and it might be legal under international laws of war.

We've massed forces for an attack, attacked their ships, violated their airspace with combat aircraft (that's today), and extensively and publicly threatened them. They'd be in their legal rights to strike preemptively, including possibly a decapitation strike (this is why the Dubya administration kept repeating the term "preemptive strike", even though it was obviously nowhere near applying in the case of Iraq—it was a way of asserting its legal basis)

[edit] As thereisnospork points out in a sibling comment, however, this doesn't mean it'd be a good idea.