The memo was brief—just a few hundred words. The memo was polite. But for President Lyndon Johnson and his NATO allies, it read like a slap in the face.

“France is determined to regain on her whole territory the full exercise of her sovereignty,” wrote French President Charles de Gaulle. The country intended to stop putting its military forces at NATO’s disposal and intended to kick NATO military forces—and those of NATO members—off of its land.

In short, de Gaulle had just done the unthinkable: pulled the plug on a crucial part of NATO.

De Gaulle’s 1966 decision to withdraw France from NATO’s integrated military command sent shock waves through NATO’s member states. It was a reminder of the fissures within the North Atlantic Treaty Organization—and a challenge to its very existence. Could NATO survive without a member state’s participation in the very military agreements it was founded on?

NATO itself had been founded out of fears of nuclear war, and during the 1950s, the alliance began to formalize its military agreements. Fearing that a war with the USSR would require a formal military structure on the part of NATO, member states decided to create its own joint military command.

The integrated military structure, as it was named, created a framework for NATO military responsibilities and helped dictate just how member states would contribute in case of military action. It was created just as the Cold War heated up, with revelations that the USSR was positioning nuclear weapons in Cuba aimed directly at the United States and increasing tension around the Iron Curtain, as the military and ideological boundary between Western Europe and Soviet-bloc countries was called. And as world affairs became even tenser during the 1960s, the strain was reflected within the NATO alliance.

“The ’60s saw NATO more divided and under greater stress than at any time since its creation in 1949,” explained Jamie Shea, NATO’s Deputy Assistant Secretary General for Emerging Security Challenges, in a 2009 lecture. And for France especially, said, Shea, “there was a real sense of not being treated equally.”

Over the years, France had come into conflict with nearly all of its NATO allies, especially the United States and Britain. Though all three countries had founded the steering group from which NATO was born, France had soon fallen out of the driver’s seat. French president Charles de Gaulle still resented what he saw as the United States’ abandonment during the 1956 Suez Crisis, when the U.S. effectively forced France to withdraw its forces from the area around the Suez Canal during a conflict over its nationalization by Egypt. And he valued French military independence—something he felt could never be achieved within the context of the alliance.

Frustration mounted even more when de Gaulle suggested that France, the United States and Britain be put on equal footing within NATO in terms of nuclear strategy. The proposal failed, and as a result de Gaulle began slowly reducing French participation in NATO. He withdrew France from the Mediterranean fleet and refused to store nuclear weapons from other countries on French soil.

The situation reached a boiling point by 1963, when the U.S. and France clashed over a plan to have NATO nations man a North Atlantic nuclear fleet. De Gaulle and his military had planned their own North Atlantic nuclear fleet, and withdrew France’s participation as a result. Then, in 1966, de Gaulle struck a final blow. He announced that he was withdrawing France from the integrated military structure and that all foreign forces had to leave France.

It was the first major crisis faced by the alliance, and it shook member nations deeply. On both sides of the Atlantic, politicians and pundits mused on the best way to proceed forward. President Johnson castigated De Gaulle in a strongly written letter; Dwight Eisenhower proposed that NATO appoint a French commander. For The New York Times’ editorial board, there was only one solution: for the U.S. to stand down. “Bonn and London, in turn, must make it clear to Washington that continued American predominance cannot save NATO, but only destroy it,” they wrote. “The Atlantic alliance can only be restored in one way, through restoring the unity of Europe.”

Yet the alliance lived on. The withdrawal forced all member states to remove their French bases, and NATO itself had to move its military headquarters from France to Belgium. But France did not withdraw from the political alliance of NATO, and made behind-the-scenes assurances to the United States—the Lemnitzer-Ailleret Agreements—that it would support NATO in the case of nuclear war in Europe.

It took 43 years for France to change course. By the time Nicolas Sarkozy announced that France would rejoin the military portion of the NATO alliance in 2009, the USSR no longer existed, the Cold War was over and France had participated in NATO peacekeeping operations in the Balkans and Afghanistan.

“We send our soldiers onto the terrain, but we don’t participate in the committee where their objectives are decided?”said Sarkozy. “The time has come to end this situation. It is in the interest of France and the interest of Europe.” France was accepted back into the fold—a powerful reminder that the alliance has so far managed to sustain itself despite vehement differences among its member states.