The early discoveries led to a few controversies not only concerning territorial claims but also concerning geographic nomenclature. The struggle for national influence was especially acute in the slender peninsular landmass south of the Scotia Sea that became known as O’Higgins Land (Tierra O’Higgins) to Chileans and San Martin Land (Tierra San Martín) to Argentines, named for national heroes who helped in gaining independence from Spain. To the English it was known as Graham Land, after a former first lord of the admiralty, and to Americans as Palmer Peninsula, after the sealer and explorer Nathaniel Palmer. By international agreement, the region is now known simply as the Antarctic Peninsula.

The first half of the 20th century is the colonial period in the history of Antarctica. Between 1908 and 1942 seven nations decreed sovereignty over pie wedge-shaped sectors of the continent. Many nations—including the United States, the Soviet Union, Japan, Sweden, Belgium, and Germany—carried out Antarctic exploration without lodging formal territorial claims, even though claims may have been announced by some of their exploratory parties. The U.S. government, for example, has never taken up the claims made in 1929 by Richard Byrd’s expedition in the Ford Ranges of Marie Byrd Land (an area presently unclaimed) nor those made by Lincoln Ellsworth on aerial landings on November 23, 1935, in Ellsworth Land (an area now claimed by Chile) and on January 11, 1939, in the American Highland near the Amery Ice Shelf of East Antarctica (an area now claimed by Australia). The German Antarctic Expedition of 1939 aerially photographed an extensive segment of Princess Astrid and Princess Martha coasts of western Queen Maud Land and, dropping metal swastikas over the region, claimed it for Nazi Germany (the area is now claimed by Norway). Other claims were transferred, such as that made in 1841 by James Clark Ross, who, after discovering and naming the coastal Ross Sea region after Queen Victoria, claimed it for the British crown; the area was later transferred to, and is now claimed by, New Zealand.

After the French claim of Adélie Land caused Americans to demand retaliatory action, the United States’ official position on the sovereignty issue was announced in 1924 by Secretary of State Charles Evans Hughes:

It is the opinion of this Department that the discovery of lands unknown to civilization, even when coupled with a formal taking of possession, does not support a valid claim of sovereignty, unless the discovery is followed by an actual settlement of the discovered country.

This policy has been reiterated many times since.

Few combative activities have marred the history of Antarctica. World War II brushed the continent only lightly when Nazi commerce raiders used its nearby seas. The threat of increased activity, however, prompted British warships to keep the northern Antarctic Peninsula under surveillance. On one visit to Deception Island in January 1943, they discovered that Argentine visitors had been there the year before, leaving a brass cylinder with notice of claim to the peninsular region. The British obliterated the Argentine signs left there, hoisted the Union Jack, posted a notice of crown ownership, and returned the cylinder to the Argentine government. Reaction was swift. In London, growing concern that the territory might be lost and that a pro-German Argentine government might control both sides of the vital Drake Passage linking Atlantic and Pacific sea routes resulted in a secret military plan, code-named “Operation Tabarin,” to establish a base on Deception Island for closer watch. When the British returned to the island in February 1944, they found their earlier sign gone and an Argentine flag painted in its place. This they soon replaced with their own flag, and their base was established to back up the British claims to the region. Several other stations were built, and, with the conclusion of the war, the United Kingdom decided to maintain a continued presence in Antarctica.

Argentina and Chile both increased their activities to back up their claims to the Antarctic Peninsula as a result of the British occupancy. (Chile had expressed a claim in 1940.) The Argentines had maintained a weather station in the South Orkney Islands continuously since 1903, and after 1947 they and the Chileans constructed bases at several sites. In addition to using effective occupation, Chile and Argentina used arguments based on historical treaties and geographical contiguity in staking their claims, but these arguments had little international legal currency at the time. With the coming of the U.S. Ronne Antarctic Research Expedition (RARE) in 1947–48 to the old U.S. Antarctic Service East Base camp on Marguerite Bay, the peninsula protagonists—British, Argentine, and Chilean—became concerned that the United States might restore its claims.

Military violence has flared on two occasions in the region and in both instances has involved Argentina and the United Kingdom. The first incident took place in 1952 when the Argentine navy used small-arms fire to chase back to its ship a British meteorological party that had landed at Hope Bay (at the northern end of the Antarctic Peninsula). The matter was resolved when the Argentine government agreed not to interfere with the party. The second, much more serious confrontation took place in 1982 in the Falkland Islands, a British colony that is also claimed by Argentina (called the Islas Malvinas by the Argentines). Argentine forces invaded the Falklands and South Georgia Island in early April. The British responded by sending a military task force, reoccupying the islands, and forcing the Argentines to surrender on June 14 (see also Falkland Islands War).

By the mid-1950s many nations had active Antarctic interests, some commercial and some scientific but generally political. In 1947–48 Australia had established stations on Heard and Macquarie islands and in 1954 built Mawson Station on the mainland coast of Mac. Robertson Land as a basis for its vast territorial claim. South Africans raised their flag over Prince Edward and Marion islands. France had established permanent bases by 1953 in the Kerguelen and Crozet islands and surveyed much of the Adélie Land coast. In 1955, with icebreaker aid, Argentina established General Belgrano Station on the Filchner Ice Shelf. A profusion of British, Chilean, and Argentine bases had been built in such proximity to one another on the peninsula and nearby islands that their purpose seemed more for intelligence activities than for science. The international Norwegian-British-Swedish Expedition of 1949–52 carried out extensive explorations from Maudheim Base on the Queen Maud Land coast in the territory claimed in 1939 by Norway. The United States had shown little interest in Antarctica since the Ronne expedition and the U.S. naval “Operation Windmill,” both in 1947–48 (the latter expedition was to obtain ground checks on the aerial photography of the previous season’s “Operation Highjump”), but it continued its policy of nonrecognition of any claims. The Soviet Union had shown little interest, other than in whaling, in Antarctica since Bellingshausen’s pioneer voyage. On June 7, 1950, however, the Soviet government sent a memorandum to other interested governments intimating that it could not recognize any decisions on the regime for Antarctica taken without its participation. Such was the political climate on the continent during the organizational years for the coming International Geophysical Year (IGY) of 1957–58.