What's the history of the Panama Canal, and why is Trump ...

President-elect Donald Trump has suggested the US should retake the Panama Canal, an idea that was immediately rejected by the government of Panama, which has controlled the passage for decades.

In social media posts and remarks to supporters, Trump accused Panama of charging the US "exorbitant rates" to use the canal and hinted at growing Chinese influence over the crucial waterway.

"The fees being charged by Panama are ridiculous, especially knowing the extraordinary generosity that has been bestowed to Panama by the US," Trump wrote on Truth Social on Saturday.

The US-built canal was opened in 1914 and controlled by the United States until a 1977 agreement provided for its eventual handover to Panama.

The canal was jointly operated by both countries until the Panamanian government retained full control after 1999.

Speaking to a crowd of young conservatives in Phoenix on Sunday, Trump said if the spirit of that agreement was not followed, "then we will demand that the Panama Canal be returned to the United States. So, to the officials of Panama, please be guided accordingly."

It's not clear how seriously Trump is taking his threat to reclaim control over the canal, though the weekend was not the first time he has said the US was getting a raw deal.

The president-elect has not clarified how he would force a sovereign, friendly country to cede its own territory.

And the Panamanian government wants nothing to do with Trump's suggestion.

"As president, I want to express precisely that every square metre of the Panama Canal and its adjacent area belong to PANAMA, and will continue to be," President José Raúl Mulino said in a statement on Sunday.

"The sovereignty and independence of our country are not negotiable," he added.

A fraught and deadly history

Before the canal's completion, ships traveling between the east and west coasts of the Americas would have to sail around Cape Horn, on the southern tip of South America, adding thousands of miles and several months to their journeys.

Creating a passageway that would shorten that trip had been an elusive goal of several empires that had colonies in the Americas.

In the early 20th century, President Theodore Roosevelt made the completion of a passageway a priority.

The territory was at the time controlled by the Republic of Colombia, but a US-supported revolt led to the separation of Panama and Colombia and the formation of the Republic of Panama in 1903.

The US and the newly formed republic signed a treaty that year that gave the US control over a 16km strip of land to build the canal in exchange for financial reimbursement.

The canal was completed in 1914, cementing the US' status as an engineering and technological superpower, but it came at an enormous human cost.

About 5600 people were estimated to have died during US construction of the canal.

The canal's practicality was demonstrated during World War II, when it was used as a critical passageway for the Allied war effort between the Atlantic and Pacific oceans.

But the relationship between the US and Panama slowly disintegrated over disagreements about control over the canal, treatment of Panamanian workers, and questions about whether the US and Panamanian flags should be flown jointly over the Canal Zone.

Those tensions reached a peak on January 9, 1964, when anti-American riots led to several deaths in the Canal Zone and the brief severing of diplomatic relations between the two countries.

Years of negotiations for a more equitable agreement led to two treaties during the administration of President Jimmy Carter.

The agreements declared the canal neutral and open to all vessels and provided for joint US-Panamanian control of the territory until the end of 1999, when Panama would be given full control.

"Because we have controlled a 10-mile-wide strip of land across the heart of their country and because they considered the original terms of the agreement to be unfair, the people of Panama have been dissatisfied with the treaty," Carter said in remarks to Americans after the treaties were signed.

"It was drafted here in our country and was not signed by any Panamanian."

The then-president added: "Of course, this does not give the United States any right to intervene in the internal affairs of Panama, nor would our military action ever be directed against the territorial integrity or the political independence of Panama."

Not everybody supported Carter's plan.

In a 1976 speech, then-presidential candidate Ronald Reagan said that "the people of the United States" were "the rightful owners of the Canal Zone."

Tensions over the canal deteriorated again in the late 1980s under the rule of Manuel Noriega, who was removed from power after the US invaded Panama as part of the "war on drugs."

Modern troubles

Shortly after the Panamanians retained full control of the canal in 2000, shipping volume quickly exceeded the waterway's capacity.

A massive expansion project began in 2007 and was completed nearly a decade later.

But the area around the canal has been experiencing severe droughts, leading to lower water levels that hindered its ability to function properly.

Canal authorities have set restrictions on traffic and imposed higher fees to traverse the canal.

Those fees appear to form one part of Trump's issue with the canal. The president-elect on Sunday described them as "ridiculous" and "highly unfair, especially knowing the extraordinary generosity that has been bestowed to Panama, I say, very foolishly, by the United States."

Trump's other claim, that China was seeking to exert more control over Panama and the Canal Zone, was not without merit.

In 2017, Panama signed a joint communique that stressed it would not maintain any official ties with Taiwan, the self-governing democracy that China's ruling Communist Party claims as its own territory. Since then, China's influence in the area around the canal has grown.

Responding to Trump's remarks over the weekend, Mulino, the Panamanian president, said, "Rates are not a whim."

He also dismissed the idea that China exercised overt control over the canal.

"The Canal has no control, direct or indirect, neither from China, nor from the European Community, nor from the United States or any other power," Mulino said in his statement.

Trump's remarks are the latest example of the president-elect expressing his desire to obtain, or threatening to take or encroach on, territory belonging to a friendly foreign power.

Since his election in November, Trump has taunted Canadian Prime Minister Justin Trudeau by suggesting his country should be made the 51st US state.

During his first term, Trump repeatedly floated the idea of the US buying Greenland from Denmark. The island's government said it is "not for sale."

But Trump does not seem to be dissuaded. Over the weekend, the president-elect resurrected the idea while announcing his pick for ambassador to Denmark.

"For purposes of National Security and Freedom throughout the World, the United States of America feels that the ownership and control of Greenland is an absolute necessity," Trump said while announcing the pick.