First signed 8 April 1904, the Entente Cordiale laid the foundations for a stronger Anglo-French relationship. British and French troops celebrated its anniversary by swapping places.
It was the beginning of a beautiful - if complicated - friendship.
120 years ago today, the United Kingdom and French Republic signed a series of agreements aimed at resolving conflicts between the two nations and strengthening their alliance in the face of an increasingly powerful German military.
It's become a pivotal moment in European history, not only redefining the relationship between two of the world's most powerful countries, but also key to shaping the continent's political landscape throughout the 20th century.
British and French troops paid tribute on the anniversary by taking part in their traditional Changing of the Guards ceremony - but swapping palaces.
Sixteen bearskin hat-wearing British soldiers from the No 7 Company Coldstream Guards joined thirty-two members of the Gendarmerie Garde Republicaine mount guard at the Elysee Palace in Paris.
They were visited by French President Emmanuel Macron and British ambassador to France, Menna Rawlings.
Meanwhile, 32 members of the Gendarmerie’s Garde Républicaine marched with 40 Guardsmen from the F Company Scots Guards at Buckingham Palace. On behalf of King Charles III, they were greeted by the Duke and Duchess of Edinburgh alongside the French Ambassador.
It was the first time a country from outside the Commonwealth has taken part in the Changing of the Guard ceremony in London.
What is the Entente Cordiale?
Relations have always been rocky between England and France, to say the least - two headstrong countries with powerful global influence and a history of wars (41, to be exact).
In the early 20th century, there were various conflicts still simmering, especially regarding colonial disputes in North Africa. The Entente Cordiale put these to bed, establishing diplomatic harmony between the countries.
It also became somewhat of a necessity, with growing fears over Germany's rising militant powers and founding of the Triple Alliance, a secret pact with Austria-Hungary and Italy.
Signed a decade before World War I began, the Cordiale was the start of a powerful alliance that created a template for unity and peace across Europe.
In 1949, England and France - along with twelve other countries from Europe and North America - founded NATO (North Atlantic Treaty Organization) to protect against the Soviet Union's nuclear threats.
Perhaps the best example of more modern-day Anglo-French unity is the Channel Tunnel. Unveiled on 6 May 1994, it's still considered one of the most impressive feats of 20th century infrastructure, completely innovating mobility between the UK and the rest of Europe.
Frenemies?
It's not all been smooth sailing, of course. Like any relationship, France and England have had their differences, the latter (and former Prime Minister Boris Johnson in particular) complicating matters by leaving the European Union in 2020.
It was a classic case of an anxious avoidant causing abandonment issues in their partner - and a whole lot of financial mess.
But 120 years later, they're still together and working through their issues.
In a positive symbolic move, King Charles III and Queen Camilla made their very first state visit to France last year.
While there (and reportedly feasting on blue lobster), the King referenced the importance of the Entente Cordiale in a statement:
"The connections between our people are myriad, and represent the lifeblood of our Entente Cordiale, which was inspired by my great great grandfather, King Edward VII."
He continued, "It is incumbent upon us all to reinvigorate our friendship to ensure it is fit for the challenges of this, the 21st Century."
Hear, hear, and viva la besties.