Over the last few months, I have been going carefully through my parents’ wartime papers. It’s all there in the letters and forms and telegrams and official stamps. The desperate failed attempts at escape, the scramble to get documents that might let you out, the scramble for documents that might let you in, the trails that go cold, the miracles that set you free.

It has all seemed so very real, so very immediate, these last few days, watching the scenes at Kabul airport as Afghans tried desperately to flee. Their experience seemed a version of the Jewish experience.

But it was not just the airport scenes that had that impact. It was also the withdrawal. The withholding of American power and what that withholding means. That too is part of the Jewish experience.

Pearl Harbor and the American entry into the war was the decisive event of the 20th Century. It brought to an end a policy of isolationism and doomed Hitler to defeat. It led directly to decades of American involvement in Europe, its broader policy of intervention and its support for the state of Israel.

Without it, Hitler might possibly have triumphed and the Jewish people might have been utterly annihilated. There would have been no Israel because there wouldn’t have been any Jews to go there. Franklin Roosevelt knew what was at stake for mankind but many of his countrymen did not, or did not care. Pearl Harbor forced them to care. And the difference between America withholding its power and exercising it was starkly demonstrated.

We have called the period shaped by that decision the “postwar era”. It has also been called Pax Americana. Jews have been protected by American power, Western interests and by the memory of the Holocaust. But it may now be that this era is over. We are living in the post-postwar world.

What is happening in Afghanistan is not a regional decision to end a conflict that wasn’t going well. It is part of a long-term decision by the United States to revert to its traditional foreign policy, the one it had before Pearl Harbor. It is a strategic and emotional retreat. A choice not to be the world’s policeman. A choice that foreign entanglements are not in the interests of Americans.

Jews are less safe as a result. We know now, as probably we should have appreciated for some time, the limits of American guarantees and of its willingness to make sacrifices. Insofar as Israel’s security depends on American goodwill and commitment, a warning has now been issued about how much such goodwill and commitment can be trusted.

Alongside these limits is the change in perspective that time has brought. The dawning understanding of the Holocaust helped shape Western liberal politics in the postwar era. But now we have reached post-postwar. As the survivors pass into history, memories fade. And the Holocaust seems less an immediate crime and a present threat and more an historical event. Like the Tudors.

There are many reasons why some on the left have turned against Israel, but one is the simplest of all: forgetfulness. People such as Harold Wilson and Jim Callaghan had seen what had happened to the Jews and knew why Israel was needed. When modern left critics talk of the victim status of Palestinians they seem almost entirely ignorant or uninterested in the reason why millions of Jews sought a homeland. We have, as Jews, become so used to the conventions of the postwar era — Nato, the power of the Munich analogy, the memory of the Holocaust, Western unity — that we may not have given enough thought to how dependent we have become on them. And about what we do now.

We may have thought that we were moving ever upwards, towards international law and a world brotherhood and the triumph of liberal ideas. When the Berlin Wall came down it felt like that. It felt like we understood what “never again” meant and that the world might rally against a repetition of genocide. All this seems very naive now.

One Jewish friend on seeing the airport scenes sent me a text saying of the Americans: “You can’t see these people bombing the rail tracks.” And I knew what he meant. I suddenly felt more alone.

Daniel Finkelstein is Associate Editor of The Times