President Obama heads to Cuba in two weeks, eager to make more travel and trade concessions to its Communist regime. But members of Congress worry that Mr. Obama is so eager to befriend Cuba he’ll cave to any demand — including handing over the US Naval base at Guantanamo Bay.

The chairman of the House Foreign Affairs Committee, Ed Royce, introduced a bill last week to bar such a giveaway. Mr. Royce’s bill is likely to face a presidential veto. As a practical matter, the surest way to prevent the president from surrendering Guantanamo is to keep its military detention facility loaded with prisoners. The president can’t relinquish it until it’s empty.

America has controlled the 45-square-mile property at the eastern end of Cuba for more than a century. When the Castro brothers seized power in 1959 they demanded Guantanamo back. They failed then, but they’re trying again, exploiting the fact that the United States now has a president inclined to placate America’s foes.

When Mr. Obama initiated a diplomatic thaw with Cuba’s Communist regime, Raul Castro said surrendering the base had to come before “normalizing relations.”

Threats like that tend to make Mr. Obama go wobbly. This standoff could turn into a repeat of his Iran nuke deal.

Already, we’re hearing mixed signals from the administration. Defense Secretary Carter says “it’s a strategic location” and “we intend to hold onto it.” But last summer, Secretary of State Kerry said “we understand Cuba has strong feelings about” getting Guantanamo back and “I can’t tell you what the future will bring.”

The toughest members of the president’s own party worry he needs to re-examine his naïve approach to Cuba’s regime. Senator Menendez, a New Jersey Democrat and a son of Cuban immigrants, worries the president is being duped. The Castros are responding to Mr. Obama’s concessions by cracking down on political dissidents. In January alone, the regime threw 1,414 more dissidents into jail.

So why is Mr. Obama about to hand the Castros a public-relations coup: an official visit by a serving president of America — the first in nearly 90 years?

The Castros have made it clear they want America’s surrender of Guantanamo to be on the visit’s agenda. Mr. Obama’s deputy national security advisor, Ben Rhodes, has said he’s “sure that will be part of the discussion.”

Does the president have the authority to cede Guantanamo? This military installation is two-thirds the size of Washington, DC. It overlooks one of the hemisphere’s most trafficked sea-lanes, which connect the East Coast to Central and South America. General Kelly, who is in charge of the Southern Command, calls Guantanamo “indispensable to the Departments of Defense, Homeland Security, and State.”

Despite the installation’s importance, constitutional law is murky on whether Obama can give it away without Congress’ approval. America leases Guantanamo Bay through a treaty signed with Cuba long before the Castros came to power. Entering into a treaty requires the advice and consent of the Senate. Ending a treaty is a different story. The Constitution says nothing about it. A number of presidents have pulled out of treaties without consulting Congress.

It’s clear the treaty allows the United States to “abandon” Guantanamo if it chooses. Which is why such importance suddenly attaches to the notion that the nation can’t walk away while there are prisoners there. That could be what saves the United States from Mr. Obama’s willful surrender of the base.

Congress has hobbled Mr. Obama’s ability to move any more Guantanamo prisoners to foreign countries (slipping that restriction into a defense-spending bill). That leaves one option: moving these dangerous terrorists to the American mainland by executive order. Even the liberal Washington Post warned “such a move would ignore the repeatedly expressed will of Congress.”

But knowing Obama’s fondness for appeasing America’s foes and his loathing of symbols of US global power, it’s still possible he’ll pull a fast one and return from Cuba humming, “Adios, Guantanamo.”

Ms. McCaughey is a senior fellow at the London Center for Policy Research.