President Donald Trump has put defense spending levels among NATO-member countries front and center this week in Brussels, where he has repeatedly berated America's NATO allies for not meeting an agreed-upon goal for each country to spend 2 percent of its annual gross domestic product on defense.

And while Trump is the first president to let defense spending levels dominate his approach to NATO, Trump is not the first president to pressure NATO countries to increase the amount they spend on their national defense.

Both former presidents George W. Bush and Barack Obama regularly expressed frustration with NATO member governments for not spending more of their domestic budgets on defense.

In 2006, then-president Bush used a NATO summit in Latvia to pressure allies to increase their defense spending at the height of the U.S.-led NATO military campaign in Afghanistan.

Two years later, he used his final NATO summit to do the same thing. "At this summit, I will encourage our European partners to increase their defense investments to support both NATO and EU operations," Bush said at the opening of the 2008 summit in Bucharest, Romania. "America believes if Europeans invest in their own defense, they will also be stronger and more capable when we deploy together," he said.

And despite the many differences between Bush's foreign policy and that of Obama, his successor, one thing the two leaders agreed upon was the need for more defense spending from NATO allies.

For Obama, the issue of NATO defense spending became especially important during his second term, when Russia's arming of separatists in Ukraine and subsequent annexation of Crimea in 2014 stunned the West.

"If we’ve got collective defense, it means that everybody’s got to chip in, and I have had some concerns about a diminished level of defense spending among some of our partners in NATO. Not all, but many," Obama said at a press conference in Brussels in March 2014, less than a week after Russia declared that Crimea was now a Russian state.