Many pundits are mystified by Donald Trump's foreign policy. That's because they focus too much on what he says ... and even more on what he tweets.

But Trump uses his tweets and off-hand comments to drive the news cycle and poke his critics. They are not meant as policy pronouncements.

Knowing what the White House is really up to requires looking at the formal proposals and policy statements coming out of the administration.

And when it comes to understanding the totality of the president's foreign relations policy, the newly released National Security Strategy is a good place to start.

Yes, it's a different way of reading the tea leaves. While modern presidents have regularly churned out security strategies, analysts have grown accustomed to not taking them very seriously. That's because most presidents have used them as aspirational manifestos or public relations ploys.

But Trump's national security strategy is different. For starters, it is much longer and much meatier. It lays out — in as much detail as possible in an unclassified document — exactly how the administration plans to engage the world. It reveals Trump's worldview and lets both our friends and competitors know exactly where they stand.

The president's critics — convinced that he has no worldview, much less a grand strategy — dismissed the document out of hand. In doing so, they missed the boat.

Yes, for a world leader, Trump presents a tremendously unconventional public face. But behind that face is the mind of a serious and systematic man. He possesses a remarkable clarity of vision — one that enabled him to become a business success, a national celebrity and, ultimately, the president of the U.S.

Put another way, Trump likes to know where he is going. And his national security strategy tells us where he wants to go as a world leader.

That strategy, simply put, is to split the difference between George W. Bush and Barack Obama.

While Bush's aggressive foreign policies made as many problems as they solved, Obama's timorous, lead-from-behind approach created a leadership void that anti-American regimes rushed to exploit, creating even bigger problems.

Trump looks to stand up for American interests while not trying to reshape the world in America's image.

Given the number of serious security challenges facing the U.S. — from North Korea's Rocket Man to a restive Russia, from rising China to unsteady Iran — this strategy makes sense. But there's a problem.

For it to work, the U.S. must demonstrate the willingness and the capability to protect America's interests. In addition to diplomatic skill, this requires a significant military presence in all parts of the world where America is pressed. And, right now, we don't have that much military.

Under four successive presidents, the U.S. under-invested in its armed forces. Even when spending bumped up under Bush, most of the increase went to fight the wars in Iraq and Afghanistan.

Under Obama, it became abundantly clear that America had worn out its military. Yet, he cut the force more and cut defense spending, year after year, even more.

The Heritage Index of U.S. Military Strength objectively grades the level of American military power in each service according to its ability to perform the most critical missions.

Over the last half-decade, the index shows a steady decline in both military capacity and capability. Readiness is lower, equipment isn't being modernized fast enough, and the military is simply too small to meet its global responsibilities.

Unless Trump can negotiate sustainable budgets that adequately fund the military and rebuild force over time, he won't have the military he needs to support his strategy.

That's certainly not the legacy the president envisions for himself. More importantly, failing to replace aging warships, planes and tanks would severely jeopardize both American prosperity and global peace.