Sen. Bernie Sanders recently declared that he would oppose the defense budget. His reasons: It’s bloated, and the money would better be spent elsewhere.

The Vermont independent has every right to argue that money used to fund the military would be better spent on other priorities—in his case, climate change subsidies and welfare programs. But it is important to start these budgetary arguments from a common base of facts.

There is no question there is waste in the defense budget. A recent Heritage Foundation report identified a long, detailed list of suggested cuts and reforms to the defense budget.

That being said, some defense priorities merit added funding. Rather than acknowledge this, Mr. Sanders relies on several broad assertions about the U.S. defense budget that fall apart under a bit of scrutiny.

For example, he claims the U.S. defense budget is as big as those of the next 10 countries combined and more than three times what China spends on defense. Neither assertion is true.

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The Washington Post gave then-President Barack Obama one “Pinocchio” in 2012 for making the same claim, noting that China isn’t honest about its real defense spending (shocking), that purchasing power parity reduces the gap between the U.S. and China (and others) by a huge amount, and that raw numbers in U.S. dollars are misleading when used as an indicator of military strength.

Indeed, recent analysis shows that the “next 10 countries combined” canard may be even more wrong than most people think.

Estimates at the University of Western Australia have pegged the Chinese defense budget at nearly $500 billion when using a purchasing power parity analysis—roughly twice the official number reported by the Chinese government.

In June, Senate Armed Services Committee member Dan Sullivan, Alaska Republican, revealed that internal U.S. government estimates project the actual size of the Chinese defense budget to be in the neighborhood of $700 billion, just behind the U.S. defense budget’s most recent total of $816.7 billion.

For added context, remember that China’s defense spending is focused almost entirely on its immediate neighborhood, whereas U.S. defense spending must support our global commitments.

U.S. military preeminence isn’t preordained. A change in the regional balance of power is a real possibility if Chinese defense investments produce a military as large and as capable as ours.

Mr. Sanders also raises the ongoing audit of the Department of Defense, ignoring the difficulties inherent in auditing such a huge organization—one with a budget of over $800 billion. Moreover, many of the issues with defense contracting costs and its civilian workforce inefficiency stem from regulations imposed by Congress itself.

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If lawmakers want the Defense Department to behave more like a private company in financial terms, it could help by cutting some of the red tape that unnecessarily ties the Pentagon’s hands in dealing with contractors and makes it harder for the department to hire and fire civilian workers.

Curiously, while Mr. Sanders balks at funding the military at the level needed to complete its many missions, he has little hesitation about using the U.S. military abroad. He voted in favor of using military force against Yugoslavia in 1999 and the war on terrorism in 2001, and he supported Mr. Obama’s decision to send troops and military aid to Syria.

Most recently, he has voted in favor of sending weapons to Ukraine and confronting the Russians in Europe. About the only exception he is willing to make is for China; he has consistently downplayed the threat posed by Beijing and attempted to hold up the funding needed to counter it.

Congress asks the military to do quite a lot, and the Defense Department doesn’t have final say in how it spends its money or where it operates. Lawmakers want the military to be able to operate on every continent: to deter China and Russia; to conduct counterterrorism operations in the Middle East, foreign internal defense missions in Africa, counternarcotics operations in Latin America, etc.

The least Congress can do is provide funding commensurate with and tailored to the strategic priorities that the government has tasked our troops with fulfilling.