One of the few pledges that Donald Trump made during the campaign that brought joy to the hearts of national security conservatives was his promise to rebuild the U.S. armed forces from the damage suffered under the sequestration budget cuts. He called for increasing the army from 450,000 active-duty soldiers to 540,000, the Marine Corps from 182,000 marines to 200,000, the Navy from 272 deployable ships to 350, and the air force from 1,141 combat aircraft to 1,200. This is a great idea. The U.S. armed forces need to get bigger to counter the rise of Russia, China, Iran, North Korea, and other threats.

The fact that Republicans control both houses of Congress makes it more likely that this vision can be realized, but it is far from a sure thing. Trump did not spell out how he would pay for his ambitious spending plans. Mackenzie Eaglen of the American Enterprise Institute noted that President Obama was already planning over the next four years to spend $113 billion more than the sequestration caps allow. Trump’s spending plan would require a minimum of another $250 billion to $300 billion extra over four years.

Where is the money coming from? During the campaign, Trump claimed that the entire amount could be generated by simply eliminating “waste, fraud and abuse” at the Pentagon. If only. The reality, as old Washington hands know, is that one person’s waste is another person’s vital program. Each line-item in the defense budget has its defenders and won’t be easily cut. Some wasteful spending can be squeezed out but nowhere near enough to pay for Trump’s defense buildup.

Congress would have to appropriate more money for defense, but while this will find favor with defense hawks, it is sure to meet resistance from budget hawks. In his election victory speech, the president-elect didn’t mention defense spending but did tout his plan to spend $1 trillion on rebuilding infrastructure.

And then there is Trump’s other priority: tax cuts. Independent budget analysts suggest that Trump’s tax cut plans could cost the Treasury as much as $10.8 trillion in lost revenues over ten years. Some of those losses could be offset by goosing economic growth, provided Trump doesn’t enact job-killing tariffs, but not all. Trump does not have comparable budget-cutting plans. In fact, during the campaign, he ruled out cuts to entitlement programs like Social Security and Medicare.

If all of Trump’s plans are implemented, the Tax Foundation found, “Including the costs of additional federal borrowing from Mr. Trump’s plans, the national debt would rise by $5.3 trillion over a decade relative to current policy, pushing the debt-to-GDP ratio to 105 percent, which is significantly higher than either Mrs. Clinton’s policies or the current trajectory for the debt.”

I find it hard to believe that a Republican-dominated Congress will accede to such a massive debt increase without a fight. Something will have to give. Not long ago, Trump was praising Pentagon budget cuts. Will Trump fight to enact the defense buildup to which he committed relatively late in the campaign? It’s unclear, and that makes it all the more important to appoint a defense secretary who will work to ensure that the Defense Department has the funds it needs to keep up with America’s enemies.