The United Kingdom’s ambassador to the United States, Christian Turner, has called the four-day visit by King Charles III to the US an effort to “renew and revitalise a unique friendship” between the two allies.

The visit, which began on Monday, comes at a tense period in US-UK relations as US President Donald Trump continues to publicly criticise British Prime Minister Keir Starmer over his stance on the US-Israeli war on Iran, accusing him of not assisting Washington in the fight against Iran or helping the US reopen the strategic Strait of Hormuz.

Days after the war began – and after Starmer initially refused to allow US forces to use UK military bases for strikes on Iran – Trump described the British leader as “not Winston Churchill”.

Despite the US gaining independence from Britain in 1776 and fighting the War of 1812 against it, the two countries have for the most part remained firm allies and have further solidified their relationship over the past century into what is often called the “special relationship”.

Nevertheless, their ties have faced periodic strain, even when they have agreed to align on larger issues.

Here is a timeline of the highs and lows in this “special relationship”:

1940-1944: World War II

One of the most significant alignments between the countries took place during World War II when London and Washington closely coordinated the war effort with the Soviet Union for the Allies against the Axis powers, led by Nazi Germany.

The two countries came together specifically on what became the “Germany first” strategy, in which they prioritised defeating the Nazis over combating imperial Japan.

In that same period, US President Franklin D Roosevelt also signed into law the Lend-Lease Act in March 1941. It provided war and military supplies to the UK and others in the Allied camp, even before the US formally joined the war in December 1941.

1956: Suez Canal crisis

France and the UK quietly coordinated with Israel to launch a joint invasion of Egypt after President Gamal Abdel Nasser moved to nationalise the Suez Canal in 1956.

Furious at having been kept in the dark and worried that the Soviet Union, then a close partner of Egypt, might be pulled into the fighting, US President Dwight Eisenhower backed a United Nations resolution condemning the assault.

Washington also warned it would withhold crucial financial support from its European partners, and within days, the fighting came to a halt.

The crisis was eventually brought to an end by the UN’s first armed peacekeeping force, a pioneering mission that became the model for later UN peacekeeping operations.

1982: Falklands War

When Argentina in April 1982 invaded the Falkland Islands, which had been under British control since the 19th century, the US at first refused a request by the UK for military assistance.

The US, which was also an ally of Argentina, wished to stay out of the dispute. Instead, US President Ronald Reagan urged UK Prime Minister Margaret Thatcher to negotiate a peace deal and recommended agreeing to joint control of the islands.

Thatcher disagreed, and the UK took the islands back after a 10-week war, in which the US did provide logistical support to the UK.

1994: Northern Ireland

US President Bill Clinton granted Gerry Adams, leader of Sinn Fein, a Northern Ireland political party, a 48-hour visa to travel to the US to speak at an event in New York.

London had strongly lobbied against the visa. The UK believed Adams was a member of the Irish Republican Army, which was designated a “terrorist organisation” and had carried out bombings in the UK and Ireland.

It was later reported by Irish diplomat Sean Donlon that for several weeks after the visa was issued, UK Prime Minister John Major refused to take Clinton’s calls. In the end, however, Adams’s visit served to bring the US into Northern Ireland peace negotiations, which led to the 1998 Good Friday Agreement.

1998-1999: Kosovo War

While the Kosovo War, the conflict between ethnic Albanian separatists and Yugoslavia, is often seen as a moment of strong UK-US alignment, there were crucial disagreements between the two countries on how far military action should go to intervene.

Amid human rights atrocities and ethnic cleansing by Serb forces as well as the displacement of one million Albanians, the UK, then led by Prime Minister Tony Blair, was one of the most forceful proponents of military intervention against Yugoslavia and its Serb leader Slobodan Milosevic.

Blair repeatedly called on Clinton to take a more aggressive stance, including the possibility of deploying ground forces, but Clinton was reluctant to do so.

The US favoured a narrowly focused NATO air campaign, which took place, whereas the UK worried that airpower on its own might fail to halt Serb forces. In the end, the war was ended after a 78-day NATO bombing campaign.

2003: Invasion of Iraq

The Iraq War marked one of the closest periods of political and military coordination between London and Washington in decades. Blair strongly backed US President George W Bush’s plan to invade Iraq, committing British forces to the US-led invasion in March 2003.

Britain was Washington’s most important ally in the coalition, providing tens of thousands of soldiers and intelligence support.

Blair backed Bush at a time when other US allies like France and Canada were firmly opposed to the invasion, as were many people in the UK.

Blair’s decision triggered one of the largest protests in UK history, bringing more than one million people onto the streets of London.

2011: Libya war

After the fall and death of Libyan leader Muammar Gaddafi, US President Barack Obama revealed there were key differences between Washington and London over how to manage Libya in the post-Gaddafi era.

In a 2016 interview, Obama accused UK Prime Minister David Cameron of becoming “distracted by a range of other things” after the intervention and said he had placed too much faith in the Europeans “being invested in the follow-up”.