I’m writing a book about exactly this: The Trial at the End of History.
My answer is 1989–91. Communism collapsed, and liberalism was left without a serious ideological rival in the West. Fukuyama captured the mood perfectly: “the end of history.” Liberal democracy had won.
That victory mattered because liberalism stopped having to defend its premises. It became the background ideology of the West. From there, the autonomous individual, choice and consent increasingly took priority over borders, nation, religion, family, tradition and inherited obligation.
The borders opening were not the beginning. They were a consequence of what happened when one ideology thought it had won history.