[1] Unlike other European colonial powers such as France and Spain, Portugal did not intervene on behalf of the United States during the American Revolutionary War.

This was because of the aforementioned lack of Portuguese colonies in North America and also because of the historic alliance between Portugal and Britain, dating back to the 14th century.

In 1791, Portugal became the first neutral nation to establish diplomatic ties with the United States, leading to the arrival of an American legation headed by David Humphreys in Lisbon.

During World War II, Portugal remained neutral until 1944, when it allowed the United States to establish a military base in the Azores.

Both countries remained on the same side during the Cold War, with both Portugal and the United States becoming founding members of the North Atlantic Treaty Organization (NATO) in 1949.

Amid a wider global trend of decolonization, the Portuguese colonies of Angola, Mozambique, and Guinea-Bissau, all in Africa, began to demand independence from Portugal.

The strain in relations was caused by the United States declaring its support for these independence movements, a move which greatly angered the Portuguese government.

The revolution brought down the dictatorial Estado Novo regime that had ruled over Portugal since 1933, beginning the country's peaceful transition towards democracy and allowing Portuguese-U.S. relations to be repaired.

In 2017 the Portuguese Minister of National Defence, José Alberto Azeredo Lopes and the Ambassador of the United States in Portugal, Robert A. Sherman, signed a new deal to strengthen cooperation in search and rescue missions in North part of Atlantic Ocean.

Portugal defines itself as "Atlanticist" emphasizing its support for strong European ties with the United States, particularly on defense and security issues.

[18][19] In 2023, Portuguese Paratroopers participated with the 82nd Airborne Division in the International Exercise "Operation Toy Drop 23", in Fort Liberty, North Carolina.

[21][22] Between April and June 2024, the Portuguese Navy's submarine NRP Arpão included military personnel from the United States, Canada and Denmark in its crew during several surveillance operations on Russian ships, having even sailed under Arctic ice.