() President Donald Trump insisted Monday that he wants the Panama Canal back under U.S. control, feeding worry in Panama, home to the critical trade route and a country familiar with U.S. military intervention.

Panamanian President José Raúl Mulino rejected Trump’s vow to retake the canal, stating his nation intends to maintain control “in the name of the Republic of Panama and its people” in a statement Monday.

Here’s a look at the history of the canal and what it would take for the US to retake it:

What is the Panama Canal and when was it built?

The Panama Canal is a 51-mile man-made waterway with locks and reservoirs connecting the Atlantic and Pacific Oceans, saving ships from a 7,000-mile detour around Cape Horn at South America’s southern tip. It significantly reduces shipping time and costs, especially for time-sensitive or perishable goods.

The canal’s construction began in 1880 under Ferdinand de Lesseps, who built Egypt’s Suez Canal, but failed due to diseases, like Malaria and yellow fever, and harsh conditions, costing over 20,000 lives.

After Panama became a Colombian province, the U.S. pushed for control by sending warships and drafting a constitution granting U.S. intervention rights. Panama declared independence in 1903 and soon signed a treaty allowing the U.S. to complete the canal. The U.S. project saw 5,600 worker deaths.

Why doesn’t the US control the Panama Canal?

The Panama Canal opened in 1914, but almost immediately some Panamanians questioned U.S. control, sparking what became known as the “generational struggle” for sovereignty. In the 1930s, the U.S. renounced its right to intervene in Panama, and by the 1970s, rising costs prompted negotiations to transfer control.

The Carter administration, in collaboration with Panamanian leader Omar Torrijos, negotiated two key treaties: the “Permanent Neutrality Treaty” and the “Panama Canal Treaty.” The first granted the U.S. rights to ensure the canal’s security, while the second set a 1999 deadline for the U.S. to hand over control. Both were signed in 1977 and ratified in 1978, remaining intact even after the 1989 U.S. invasion of Panama.

While polls in the late 1970s showed roughly half of Americans opposed the transfer, by 1999, public opinion had shifted, with about half supporting it.

Why does Trump want to regain control of the Panama Canal?

Trump claims the U.S. is being “ripped off” by Panama over the 1977 canal treaty, which he calls a “foolish” giveaway.

While the neutrality treaty allows U.S. intervention if the canal’s operation is threatened, it doesn’t permit reasserting control. Experts argue there’s no legal way to recover the canal without military action.

However, Trump hasn’t said how he might make good on his threat.

Panama is also a U.S. partner in stopping illegal immigration from South America perhaps Trump’s biggest policy priority.

Can Trump order US troops to seize the Panama Canal?

Trump has suggested using military force to retake the Panama Canal, but such an action would involve significant complications. The canal spans more than 500 square miles, and Panama has a population of 4.5 million, many of whom might oppose U.S. occupation. According to U.S. Army estimates, counterinsurgency operations would require about 90,000 troops.

Additionally, any military action would be highly disruptive to global trade, as the canal handles around 6% of world commerce. It would also draw the U.S. into another land war, which Trump has criticized.

The Associated Press contributed to this report.