There may be no way for the United States to reverse the Russia’s seizure of Crimea from Ukraine. The Obama administration still has the opportunity to send a strong message to Russian President Vladimir Putin to punish Moscow for this aggression in response to the ouster of one of their stooge in Kiev by a popular uprising. Indeed, he would do well to listen to the advice of Senator Marco Rubio who outlined eight steps the U.S. should take in response to the crisis. But whether or not the president acts appropriately now, it’s probably too late to preserve the territorial integrity of Ukraine from a predatory Russia. As he did in Georgia in 2008, Putin counted on both America and Europe being too weak and irresolute to stop him from aggression carried on in his own backyard even if meant violating international law by carrying out a unilateral partition of Ukraine to either annex part of that country to Russia or, as is more likely, set up another puppet state in the strategic Crimea. At this moment, there’s little reason to believe that calculation was incorrect.

But even if we take for granted that it’s too late to save Ukraine, the spectacle of Russian aggression should provoke a re-examination of the direction of U.S. foreign policy under President Obama. It should also cause us to think again about the assumption that the American people are, as Senator Rand Paul and a growing chorus of isolationists on both the right and the left have advocated, perfectly happy to retreat from the world stage and let aggressors such as Putin ‘s Russia or Iran have their way.  The lessons of the tragedy unfolding in the Crimea are many, but surely the first of them must be that when dictators don’t fear the warnings of the leader of the free world and when America demonstrates that it is war weary and won’t, on almost any account, take firm action, to defend its interests and to restrain aggression, mayhem is almost certainly always going to follow.

No doubt there will be many, whether they call themselves realists or isolationists, who will in the coming days argue that what happens in the Ukraine is none of our business. Americans who are sick of conflict in Iraq and Afghanistan say they want no part of foreign wars or even a strong foreign policy that carries with it the chance of engaging in conflict. They may not cheer when Barack Obama speaks of “leading from behind” but they are entirely comfortable with the general drift toward retreat that has taken place in the last five years under his leadership. But, as we have seen in Syria and now in the Ukraine, there is a price to pay for such weakness and it is not one that will be paid by Bashar Assad or Putin. Nor will others who seek to test the mettle of American resolve, such as the leaders of Iran, fail to observe that the free world is led by a paper tiger. U.S. allies will draw the same conclusion.

A world in which dictators do as they like despite clear American warnings — as President Obama did first in Syria and then again this week about attacks on Ukraine — is not only a far more dangerous place. It also creates a dynamic in which every such American warning or diplomatic initiative is discounted as mere rhetoric, even if those daring to defy the United States are not so well situated as Putin is with his bold stroke in the Crimea. That is especially true with regards to the negotiations with Iran over its nuclear program.

The circumstances of the U.S. diplomatic effort to restrain Iran’s nuclear ambitions are starkly different from those in the territories of the former Soviet Union. But the basic formula of a bold rogue regime that has no reason to fear the threats or the blandishments of either the U.S. or Europe is present in the P5+1 talks. Lack of credibility in foreign policy cannot be compartmentalized in one region or particular issue. Weakness and irresolution are fungible commodities in international diplomacy. The Obama administration gave up the formidable military, political and economic leverage they had over Iran last fall by signing an interim agreement with Iran that gave Tehran what it wanted in terms of recognizing their right to enrich uranium as well as loosening sanctions in exchange for almost nothing. If the Iranians had good reason to think they had nothing to fear from the Obama administration before this latest humiliation of the president at the hands of Putin, their conviction that they can be as tough as they like with him without worrying about a strong American response can only be greater today.

It is too late to save Ukraine from the theft of its territory. But it is not too late to reverse the U.S. retreat from the world stage that has been going on in the last years. President Obama can begin to regain some of his credibility by taking a strong stand on sanctions against Russia and sticking to it. But if he doesn’t no one should be under the illusion that it won’t affect Obama’s ability to prevail in the Iran talks. The cost of Obama-style weakness and isolationism will not be cheap, either for U.S. allies or for an American people who must now understand what it is like to live in a world where no one respects or fears their government.