NATO is the alliance that has shaped Europe’s security for 77 years, but tensions with the United States have prompted the Trump administration to conduct a major review that could end with American military forces reducing and repositioning across Europe.

There is particular friction with Spain, whose Socialist prime minister refused the U.S. permission to use Spanish military bases in the recent strikes against Iran, even though U.S. taxpayers have poured billions of dollars into two Spanish military bases.

President Donald Trump has accused Spain of “freeloading” while the U.S. does the heavy lifting.

That escalated speculation that the U.S. might withdraw from NATO.

Trump has also repeatedly pressed NATO countries to commit more of their Gross Domestic Product, or GDP value, to Europe's defense.

I traveled to Madrid, the capital of Spain, and spoke with Senator Fernando Gutiérrez from the conservative party. He’s a retired General who leads the Parliamentary Assembly delegation that advises NATO. I asked if he believes Trump’s actions and rhetoric are reasonable or not.

“I think his behavior is too aggressive sometimes and that's what he presents: an aggressive posture,” Gutierrez responded. “But the principles behind that aggressive posture are understood for everyone. Everyone understands that we have to share our responsibilities.”

The U.S. helped create NATO in 1949 to counter Soviet expansion after World War II. U.S. withdrawal would be earth-shattering. It could trigger the collapse of NATO, weaken Europe’s security, and embolden Russia and China.

But NATO Secretary General Mark Rutte has emerged as Trump’s strongest defender, most famously after Trump used an expletive when comparing Israel and Iran to “two kids in a schoolyard.” After that remark, Rutte’s approach with Trump quickly became known as “Daddy Diplomacy.”

Rutte has pressed European countries to dig deeper to support the alliance and warned against drifting away from the United States in the midst of Russian aggression that led to the war against Ukraine. "Now is not the time to do it alone,” he implored earlier this year.

There’s one point even Trump’s critics concede: only because of his pressure, NATO allies met the target of spending at least 2% of their economy on their own military for the first time last year. And they’ve committed to 5% by 2035.

Most say a full US withdrawal from NATO is unlikely. But War Secretary Pete Hegseth has initiated a review called “NATO 3.0.” Analysts see it as an avenue to reduce or reposition American troops in Europe after the disappointing response regarding Iran.

Gutierrez acknowledged Trump's pressure "has been demanding because it was unbalanced and, and we have to do more,” he replied.

For more on this story, watch "Full Measure with Sharyl Attkisson” Sunday. Attkisson's most recent bestseller is "Follow the $cience: How Big Pharma Misleads, Obscures, and Prevails."