As I noted yesterday, the coming months will be decisive with regard to Iran’s nuclear program. This is an issue on which everyone would prefer if crunch time were never reached. But if a showdown must come, the timing couldn’t be more fortuitous-because it’s impossible to imagine a better geostrategic moment for military action against Iran than now.

One of the biggest concerns that opponents of military action in both Israel and America have always raised is the havoc Iran could wreak in response an attack. For Israelis, the main fear is massive missile attacks by both Iran and its allies; for Washington, the main concern is Iran’s ability to disrupt oil trade from the Gulf and attack American allies in that region.

But thanks to the Syrian civil war, the threat of Iranian retaliation has been dramatically reduced. Partly, of course, that’s because two of Iran’s principal allies, Syria and Hezbollah, are too preoccupied with that war to be able to mount serious reprisals against anyone. But even more importantly, the tremendous importance Iran attaches to Syria gives both Israel and America a powerful lever with which to restrain any Iranian reprisals.

Iran has poured billions of dollars and thousands of crack fighters-from Hezbollah, Iranian-backed militias in Iraq, and its own Revolutionary Guards Corps-into propping up Bashar Assad’s regime in Syria, because it deems Assad’s survival strategically vital. As one senior Iranian cleric explained in February, “Syria is the 35th province [of Iran] and a strategic province for us. If the enemy attacks us and wants to take either Syria or Khuzestan [in western Iran], the priority for us is to keep Syria….If we keep Syria, we can get Khuzestan back too, but if we lose Syria, we cannot keep Tehran.”

And so far, the effort seems to be working. Assad’s forces have dealt the Syrian rebels several serious blows recently; they retook the strategic town of Qusair in June and made significant gains this week in the rebel stronghold of Homs. Whether the current constellation of forces opposing Assad can reverse this tide on their own is an open question.

But there are two players who have thus far chosen to sit out the game who are definitely capable of swinging the war in the rebels’ favor: America and Israel. Both have the capacity to mount airstrikes that would destroy Assad’s air force and tanks, which have hitherto given him a huge advantage over the rebels. And both could make it clear to Iran that they would do so if its reprisals crossed any red lines.

Though America has the military might to threaten Iran directly, Syria is a much easier target, with the added bonus that any such operation would be immensely popular with its Arab allies. Hence for Washington, the ability to threaten Syria lowers the cost of deterring Iran. Israel, in contrast, lacks the military capacity to threaten Iran directly with anything bigger than a targeted operation against its military facilities. Thus for Jerusalem, the ability to threaten Syria is the difference between having almost no deterrence against Iranian reprisals and having very substantial deterrence.

That Syria’s civil war erupted when it did was pure serendipity. But knowing how to take advantage of serendipity has always been a crucial element of statesmanship.