The King has sent a “personal message of gratitude and congratulations” to Nato, as the defensive alliance marked its 75th anniversary.
Charles lauded the treaty organisation as “an antidote to years of war in Europe” and “the cornerstone of our security and the values we hold dear”, in a letter to secretary general Jens Stoltenberg.
He went on to commend Nato’s steadfast support for Ukraine and welcomed new Scandinavian member countries.
Charles also extended gratitude across the Atlantic, saying Nato was a “symbol of the enduring bond between Europe and North America”.
The anniversary comes at a testing moment for Nato, as pressure mounts on the organisation over its response to the conflict in Ukraine.
Charles wrote: “Seventy-five years ago, twelve countries gathered in Washington DC to call into being a defensive alliance that would act as an antidote to years of war in Europe, a symbol of the enduring bond between Europe and North America.
“The United Kingdom’s signature on the Washington Treaty marks our fundamental commitment to the North Atlantic Treaty Organisation (Nato) as the cornerstone of our security and the values we hold dear.”
Charles said he was pleased the alliance had grown with the addition of Sweden, which officially joined on March 7, and Finland, which became a Nato member on April 4 2023.
He said: “As we welcome the citizens of Sweden and Finland to the protection that Nato affords, our thoughts remain with the Ukrainian people.
“In the face of an unprovoked invasion, Nato has shown steadfast support for Ukraine in its courageous battle for sovereignty.”
Nato members’ commitment to Ukraine has been strong for the most part, though support from the US has wavered in recent times with a Ukraine aid bill still facing an uphill battle in Congress.
Charles praised the endurance of Nato and the UK’s prominent role within it, as he noted the relocation of the organisation’s headquarters.
He said: “The United Kingdom has been there from the start. Nato’s headquarters may have travelled from Belgrave Square in London, to the streets of Paris, and now to Brussels, but its core task endures: to protect the one billion citizens of our Nato allies.
“This vital alliance continues to thrive and adapt in the face of new threats and challenges.
“Having myself been born six months before the birth of Nato, I would like to send a personal message of gratitude and congratulations to Nato on its seventy-fifth anniversary.
“This is a moment of celebration and a reminder of Nato’s founding principle: for the preservation of peace and security.”
Prime Minister Rishi Sunak said the anniversary was a “moment to renew our historic pledge” to what he described as “the most successful alliance the world has ever known.”
He added: “The UK has been at the heart of Nato since the start. Today, faced with new threats in Europe, we continue to lead – through our incredible armed forces, our growing investment in defence, and our utter determination to stand by Ukraine – which has strengthened allied unity like never before. I say it once again: Ukraine’s rightful place is in Nato.”
Nato marked its anniversary in a ceremony prior to a meeting of the Nato-Ukraine Council in Brussels on Thursday, with Belgian and Dutch military bands playing the Nato Hymn at its headquarters.
The Nato Hymn was also performed by the Welsh Guards during the Changing of the Guard ceremony outside Buckingham Palace on Wednesday.
A larger celebration is planned for when Nato leaders meet in Washington in July.
Nato was created on April 4 1949 in Washington DC by 12 founding member countries: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States.
The organisation has since expanded to include 32 member countries, said to be bound together by common values of democracy, individual liberty, human rights and the rule of law.