Why was existentialism so popular as a philosophical and cultural movement in 1950s France?

To what extent was Italy “Americanised” after the Second World War?

To what extent was West Germany “Americanised”?

To what extent was East Germany “Russianised”?

Why was France politically unstable after the Second World War compared to Great Britain?

Why was existentialism so popular as a philosophical and cultural movement in 1950s France?