The leaders of the world's biggest military alliance are meeting in Warsaw this weekend. Key item on their itinerary: Money.

NATO has for years been pushing for more spending by its member states. The alliance increased overall defense spending for the first time in two decades in 2015, but most NATO countries still don't pay their recommended share.

At the start of the summit, NATO Secretary General Jens Stoltenberg said, "The world is a more dangerous place than just a few years ago."

Many European members -- including big economies like France and Germany -- spend less than the amount called for by NATO guidelines.

New spending data released on Monday show the U.S. shells out far more money on defense than any other nation on the planet.

According to NATO statistics, the U.S. spent an estimated $650 billion on defense last year. That's more than double the amount all the other 27 NATO countries spent between them, even though their combined GDP tops that of the U.S.

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American military spending has always eclipsed other allies' budgets since the North Atlantic Treaty Organization's founding in 1949. But the gap grew much wider when the U.S. beefed up its spending after the terrorist attacks of September 11, 2001.

NATO admits it has an "over-reliance" on the U.S. for the provision of essential capabilities, including intelligence, surveillance and reconnaissance, air-to-air refueling, ballistic missile defense and airborne electronic warfare.

The U.S. also spends the highest proportion of its GDP on defense: 3.61%. The second biggest NATO spender in proportional terms is Greece, at 2.38%, according to NATO.

U.S. presidential candidate Hillary Clinton called on other NATO members to spend more on defense. Donald Trump has gone even further, saying the U.S. should rethink its involvement in the military alliance because it is "obsolete" and other states don't pay a fair share.

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The organization is based on the principle of collective defense: an attack against one or several of its members is considered as an attack against all. So far that has only been invoked once -- in response to the September 11 attacks.

To make the principle work, all countries are expected to chip in. NATO's official guidelines say member states should spend at least 2% of their GDP on defense.

Of the 28 countries in the alliance, only five -- the U.S., Greece, Poland, Estonia and the U.K. -- meet the target.

The rest lag behind. Germany spent 1.19% of its GDP on defense last year, France forked out 1.78%.

Iceland, which doesn't have its own army, spends just 0.1% of its GDP on defense, according to the Organization for Economic Co-operation and Development.

Five other countries spend less than 1%, according to NATO's estimates for this year: Canada, Slovenia, Belgium, Spain and Luxembourg.

All member countries that fall below the threshold committed in 2014 to gradually ramp up military spending to reach the target within the next decade.

NATO is pushing hard for the 2% guideline to be taken more seriously. "We are spending more and we are spending better, but we have a very long way to go," Stoltenberg said ahead of the summit.