U.S. President Donald Trump's desire to buy Greenland has drawn ridicule, but he's by no means the first American president to have entertained the notion.

The United States is a nation built through land purchases. In fact, the U.S. has looked to the resource-rich north to expand its borders in the past — most notably in 1867, when President Andrew Johnson took Alaska off Russia's hands.

Such sales typically involve motivated sellers or conflict, and they're not so easy to negotiate these days, according to Carey Cavanaugh, a former U.S. ambassador and professor at the University of Kentucky.

"I think in general, it is outlandish," he said of Trump's desire to buy Greenland.

"Countries are very fussy in modern times over land."

Here are some of the most notable land deals that helped shape the United States.

The Alaska Purchase: $7.2 million

On March 30, 1867, U.S. Secretary of State William H. Seward and Edouard de Stoeckl, Russia's minister to the United States, signed the Treaty of Cession, which officially gave 1,518,800 square kilometres of land now known as Alaska to the United States.

Fort William H. Seward, a military post established during the Gold Rush and decommissioned in 1945, is named for the U.S. secretary of state who helped negotiate the Alaska Purchase from Russia in 1867. (Reuters)

"... the announcement conjures up in the mind of nearly every one visions of a cold, barren, and uninhabited region, converging about Behrings Straits, and celebrated only because Capt. Beechy and Sir John Franklin voyaged on its coasts," the New York Tribune wrote at the time.

Even so, the newspaper's reporter saw the deal's value, declaring it "the most important international event affecting this continent which has occurred in many years."

The sale did have some harsh critics, however, and in some circles became known as "Seward's folly." But within three decades, Alaska's value became clear when prospectors struck gold. The state, which increased the size of the United States by about 20 per cent, is now a key outpost for defence of the Arctic.

The Louisiana Purchase: $15 million

This transaction decades earlier, in 1803, when Thomas Jefferson was president, is considered one of the greatest real estate transactions in American history. France sold 2,141,920 square kilometres of land for $15 million US and nearly doubled the size of the United States in the process.

Napoleon approached the United States with the offer as he abandoned his plans for a French empire in the Mississippi Valley and turned his attention elsewhere. The Americans pounced on it.

The purchase actually included small bands of the land that later became Alberta and Saskatchewan.

The treaties that established the U.S. border with Mexico are still controversial. In 2017, Sen. Patricio Martinez of Mexico argued a surveying error after the 1853 Gadsden Purchase had led to too much land being taken by the U.S. (Brian Skoloff/The Associated Press)

Treaty of Hidalgo: $15 million

Signed in 1848, the Treaty of Hidalgo ended the Mexican-American War and saw Mexico give up about 1,359,744 square kilometres — about half of its land — for $15 million US and an end to the war.

The deal stretched the United States westward to the Pacific Ocean, where California is now, as U.S. President James Polk had wanted in the years before the war.

The Gadsden Purchase: $10 million

This purchase five years after Hidalgo, in 1853, completed the U.S.'s continental expansion. Mexico still needed money in the wake of the Mexican-American War and finalized the border between the two countries at El Paso.

Both the Gadsden Purchase and the Treaty of Hidalgo were part of "manifest destiny," an expansionist policy that had defined the Polk administration — and settled the country's main boundaries as they're known today.

The Virgin Islands were known as the Danish West Indies before the United States bought them in 1917. (Reuters)

Virgin Islands purchase: $25 million

Perhaps Trump was looking back to 1917 for inspiration on his Greenland proposition. That's when the United States bought the Virgin Islands from Denmark — then known as the Danish West Indies — for $25 million US during president Woodrow Wilson's administration.

American leaders had been looking south to buy the islands for decades, dating back to Seward's time as secretary of state in the 1860s.

In the 20th century, the United States approached Denmark to buy them out of fear Germany would annex Denmark and occupy the islands to attack ships. After initially declining the offer, Denmark later acquiesced after Secretary of State Robert Lansing suggested the United States might occupy them instead.

The Adams-Onis Treaty: $5 million

The Adams-Onis Treaty of 1819, also known as the Transcontinental Treaty, took years for the United States and Spain to negotiate. Questions lingered over the borders of the Louisiana Purchase in what is now Florida, and in 1810, a group of settlers rebelled against Spanish rule. The final deal ceded what is now Florida to the U.S.

Et tu, Greenland?

Trump is actually not the first U.S official to mull buying Greenland. It came up while Andrew Jackson was president in the 19th century. Seward also considered it around the time the United States bought Alaska.

And buying Greenland came up again after the Second World War. President Harry Truman's government actually offered $100 million US for the land in 1946. Known as the Greenland Proffer, the U.S. wanted the land for a Cold War military outpost. It didn't come to pass.

Trump may be looking at Greenland for some of the same reasons.

I promise not to do this to Greenland! <a href="https://t.co/03DdyVU6HA">pic.twitter.com/03DdyVU6HA</a> —@realDonaldTrump

But land deals are not so simple as they once were for political leaders — particularly if the country with ownership has no interest in selling.

And while Jefferson, Polk and Johnson helped define their years in office through expansion, it's not so easy now, Cavanuagh said, citing the formation of the United Nations and the signing of the Helsinki Final Act as just two impediments for Trump.

"The reality is, the world changed," he said.