The only clue we have about the setting of the poem comes in the first line, where the speaker asks about the people of Vietnam.
The Vietnam War is often called "America's longest war," even though the events all played out in Southeast Asia. The war lasted from 1955-1975 (with Americans leaving the conflict in 1973). During that time, many Americans protested the nation's involvement in the conflict by marching for peace and organizing anti-war demonstrations all over the country.
It was in this political context that Levertov wrote the poem, but it actually takes place in an imagined future inspired by the conflict. In the poem's reality, the Vietnam War has ended with the entire population of Vietnam destroyed by bombings. Nothing remains, not even their history.
While set within an imagined timeline, the poem isn't intended to seem like fantasy. Genocide and nuclear war were great realities in a post-WWII society, and in many cases still are today. By using the Vietnam War as a reference, Levertov intends to remind us that this setting could very well have been a real one.