France never left NATO, but it did change the command relationship between France’s armed forces and NATO back in 1966. It is a part of NATO today and FRance was one of NATO’s founding nations.The thing about NATO is that, it is mainly a US vehicle for domination & control of Europe under US hegemony. That is the likely leading reason why France left NATO, although remaining a kind of close ally. And you also need to ponder, what actually is the likely threat to Europe that NATO is actually defending against. Then one might get some light on the reasons for France leaving.