WASHINGTON — An essential element is missing from President Donald Trump’s plan for boosting the budgets of the U.S. military services by $54 billion in 2018. How, exactly, does the commander in chief intend to use the world’s most potent fighting force?

Beyond the threat posed by the Islamic State and other militant groups, Trump doesn’t articulate what he’s defending the country from. Defeating what Trump and his aides call “radical Islamic terrorism” doesn’t require an additional investment of tens of billions of dollars. And Trump, whose “America First” mantra suggested an isolationist approach, has viewed Russia as a potential partner, not an adversary.

Trump’s proposed defense budget of $639 billion, which includes $65 billion for ongoing emergency war-fighting, totals more than the next seven countries combined. Yet documents released Thursday by the White House provide little detail about where all the money will be spent, other than to say the goal is to rebuild an American military that Republicans have accused former President Barack Obama of allowing to fall into disrepair.

The Trump budget makes no specific mention of Iran, North Korea or China. Instead, the documents employ broad strokes to cast the $54 billion increase as “the groundwork for a larger, more capable, and more lethal joint force, driven by a new National Military Strategy that recognizes the need for American superiority not only on land, at sea, in the air, and in space, but also in cyberspace.”

Republican defense hawks in Congress immediately panned Trump’s 2018 proposal. GOP lawmakers including Sen. John McCain of Arizona, the chairman of Armed Services Committee, want at least $37 billion more than what Trump is recommending to begin to reverse an erosion in the military’s readiness for combat.

“It is clear that this budget proposed today cannot pass the Senate,” said McCain, who called Trump’s defense plan only slightly better than what Obama would have crafted if he were still in office.

And much more money will be needed over time to buy all the ships, missiles, jet fighters and more needed to replace an arsenal heavily taxed by 15 years of war, according to McCain and like-minded congressional colleagues. He envisions defense spending increasing steadily in the coming years, culminating with an $800 billion budget for the armed forces in 2022.

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Still, Trump’s 2018 plan would be welcomed at the Pentagon. Senior U.S. military leaders have warned Congress that strict caps on government spending imposed in 2011 have squeezed them so hard that beating powers such as Russia or China is far tougher than it used to be as aging equipment stacks up, waiting to be repaired, and troops don’t get enough training.

Military leaders have told Congress they’ve been forced to rob their maintenance and procurement accounts to help pay for missions in Afghanistan, Iraq and elsewhere. That’s led to critical maintenance being postponed and lengthy delays in the acquisition of new equipment, according to the Pentagon’s top brass.

“It’s a simple matter of supply can’t meet demands,” Adm. William Moran, the vice chief of naval operations, told the Senate Armed Services readiness subcommittee during a February hearing.

Despite Trump’s focus on the Islamic State group, that’s a war under control, according to top defense officials. “We’re fully ready and have shown repeatedly that we can fight today’s fight against a violent extremist organization,” Gen. Stephen Wilson, the Air Force’s vice chief of staff, told the House Armed Services Committee last month.

Trump’s budget avoids a knock-down fight with Congress over base closings. The Army and Air Force have said that shuttering excess installations would save billions of dollars. But they remain open because the GOP-led Congress has so far refused to allow a new round of base closures. Military installations are prized possessions in congressional districts.

Conservatives gave Trump’s defense plan high marks because the increase is paid for by slashing spending for foreign aid and domestic agencies that had been shielded under Obama.

But Rep. Raul Labrador, a member of the House Freedom Caucus, said he and other deficit hawks have no intention of giving the president’s plan a free pass.

“We should make sure we’re using military spending wisely like we look at every other item in the budget,” the Idaho Republican said. “I think we make a mistake as Republicans when we say it’s OK to plus-up (defense) spending and decrease everything else without really looking closely.”