Editor's note: Marco Rubio, a Republican, is a U.S. senator from Florida.
(CNN) -- For almost two years, the Syrian people have tried to free themselves of Bashar al-Assad's cruel iron fist. They have looked repeatedly to America for help. But the Obama administration has failed to robustly support the Syrian rebel movement. Instead, the administration hid behind an unsuccessful United Nations peace process.
As the conflict in Syria continued, enabling al-Assad and his forces to murder thousands of civilians, Iran and Hezbollah made matters worse by sending soldiers and weapons into Syria to prop up al-Assad. And now, jihadists are entering the country each day.
As I warned in June, the presence of jihadists will only make an eventual resolution in Syria that much harder. Coupled with the possibility of Syria's chemical weapon stockpile falling into the hands of extremists once al-Assad falls, Syria's instability is a major threat to America's interests in the region and globally.
The Obama administration's words -- that al-Assad must go and that America supports the ambitions of Syria's opposition movement -- have been the right ones. But the administration's lack of actions has disappointed the Syrian people and engendered their resentment, especially after their hopes were raised that America stood with them in their fight for a better future.
Empty rhetoric will only weaken America's credibility. It is a recipe for alienating potential allies, prolonging the Syrian conflict and imperiling America's national security.
Sensing our weakness, disinterest and failure to match our words with deeds, some countries, including our enemies, are emboldened. This is why Iran is more deeply than ever involved in arming and providing financial support to al-Assad, why Russia makes a mockery of Obama's "reset" by supplying currency for al-Assad and why Russia and China predictably continue to stifle U.S., Arab and European efforts at the United Nations.
Likewise, America's leadership void has pushed an increasing number of desperate Syrian rebels to turn to extremism. We have seen radicals seize opportunities like this before in other conflict-ridden places, and we know it never ends well.
The Syria situation is not the only sign of weakness in President Barack Obama's foreign policy.
In Afghanistan, Obama's goal of extricating American forces by a specific date leaves behind an Afghan government that might still need our assistance in establishing security and rooting out terrorists.
Instead, while the Afghans wait for us to leave, the Taliban are exploiting our vulnerabilities with brazen assaults against our troops. Recently, the Taliban pulled off a successful attack against the U.S. by destroying six Harrier jets and killing two Marines.
In Libya, the Obama administration's inaction early on in aiding the rebels' fight against Moammar Gadhafi prolonged the country's civil war by giving Islamic radicals valuable time to become involved.
Fortunately, the Libyan people resoundingly rejected extremism in their elections and in the aftermath of the terrorist attack on the U.S. Consulate in Benghazi, where Ambassador Christopher Stevens and three other Americans were killed.
Nonetheless, the administration's timidity early on helped open the door for Islamic terrorists to establish a toehold in Libya and across North Africa. As we move forward, it is critical that the United States and European Union continue to support the pro-Western Libyan government in its goal of improving the country.
Syria and Libya, of course, are very different countries with different set of challenges. Syria is much more complex -- its unraveling might have more serious repercussions in the Middle East.
While al-Assad's murderous reign will ultimately come to an end, America's ability to influence what comes afterward is diminishing by the day. It's hard to imagine a worse fate for Syria than for al-Assad to stay in power. But allowing Islamic extremists to overrun the country and enhance Iran's power would be devastating to brave Syrians who have fought so hard.
America's opportunity to protect our interests in Syria and to support people struggling against a terrible regime that has killed more than 30,000 of its own citizens is slipping away, but we can still act.
The U.S. should work with its allies to selectively arm key rebel groups, provide military guidance and continue nonlethal assistance, including intelligence sharing. We should make sure that our humanitarian assistance goes to strengthen moderate groups rather than extremist leaders.
The Syrian people have valiantly fought al-Assad with the courage of their convictions and the expectation that America would be on their side with meaningful actions.
It's time for the United States to stand up firmly to prevent what is now on track to become an epic Syrian tragedy: If and when the tyrant goes, the country will lose itself to opportunistic extremists.
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The opinions expressed in this commentary are solely those of Marco Rubio.