America's substantial support for NATO, both in money and military aid, has long been a source of frustration for U.S. leaders, and some have questioned the organization as a throwback to the Cold War era.

Republican presidential front-runner Donald Trump, in interviews this week, suggested the U.S. should scale back its role in the alliance nearly seven decades after it was launched in the aftermath of World War II. Complaining that America is spending too much money on NATO, Trump said that the financial burden has to change.

But as attacks by extremists ripped through Brussels this week, NATO rose again as a rallying point and key player in the expanding fight against Islamic State militants. The attacks underscored the need for the U.S. and its European allies to work together to counter threats ranging from groups targeting the West to the growing Russian aggression in the region.

Created in 1949, NATO has expanded from 16 members at the end of the Cold War to 28 today.

"Given this attack, I think you will see more willingness from NATO nations to join in the coalition in real and practical ways," said James Stavridis, the retired Navy admiral who served as NATO's top military commander in Europe from 2009 to 2013. As an example, he said Belgium may look to participate in coalition airstrikes against Islamic State in Iraq or Syria, and other nations may step up their contributions of military advisers or special operations forces.

Trump's criticism, however, echoed persistent complaints from some U.S. leaders, who balk at bearing as much as 22 percent of the NATO budget.

"NATO is costing us a fortune, and yes, we're protecting Europe but we're spending a lot of money," Trump told The Washington Post this week. "I think the distribution of costs has to be changed. I think NATO as a concept is good, but it is not as good as it was when it first evolved."

And while he did not fully advocate pulling out of the alliance, he said the U.S. can't afford the high price anymore.

The argument has dogged U.S. military and defense officials for years, as they pour millions of dollars of money, troops, equipment and other infrastructure into Europe. There are about 62,000 active duty U.S. forces permanently stationed in Europe, and several thousand more rotate in and out for short-term deployments for military exercises, training and other programs.

In recent years, the Pentagon reduced its permanent troop presence in Europe. But as the threats from Russia and the Islamic State grew, the military expanded its rotational deployments in a broad effort to reassure European allies and send a message to Russia that threats against NATO allies would not be tolerated.

The Pentagon has made it clear through the years that while the U.S. bears a heavy financial burden for NATO, America remains staunchly committed to its European allies and is frequently dependent on them.

"This has been a debate in the alliance going back 50 years," said Stavridis. "The allies will never spend as much as we want them to. The trade-off is, yes, we spend more than they do, but they stand with us on these operations."

The U.S., he said, counted on NATO in Afghanistan, where allies provided thousands of troops throughout the warzone. And they have done the same in Libya, Iraq, the Balkans and in the effort to contain piracy off Africa. The allies also make up the bulk of the nations involved in the airstrikes and training operations in Iraq and Syria.

"It's not perfect, but it's better than the alternative," said Stavridis, now dean of the Fletcher School of Law and Diplomacy at Tufts University. He said it makes no sense to drop out of NATO. And, rather than yelling at the allies about what they won't do, he said the U.S. should help them figure out the ways they can help.

Evelyn Farkas, former U.S. deputy assistant defense secretary, said that faced with threats from Russia and the Islamic State, NATO is improving and doing more.

"They are our allies of first resort. And in terms of return on the dollar, it's a pretty good return," said Farkas, who is a senior fellow at the Atlantic Council in Washington. "Now I think they are much more motivated. I think they are trying to find the money."

Former Defense Secretary Robert Gates delivered a brutal assessment nearly five years ago, warning that NATO faced a "dim if not dismal" future and that the U.S. would no longer carry the alliance as a charity case.

Since then, a number of European allies have moved to increase their spending on NATO, or pledged to do so in the coming years. And, faced with Russia's invasion of Ukraine's Crimea region and its support for separatists along the country's eastern border, NATO nations are participating more aggressively in their regional defense.

The alliance has fielded a NATO response force that can respond quickly to threats in the region, and recently announced plans to expand it. And just last month, Jens Stoltenberg, NATO's secretary-general, announced that the alliance ordered three warships to the Aegean Sea to help end the deadly smuggling of asylum-seekers across the waters from Turkey to Greece.