The poem is about change and momentary existence. It says the green of spring is actually gold. This precious time is fleeting; it is Nature’s “hardest hue to hold.” This idea of not being able to hold on to things is repeated throughout the poem. The third and fourth lines of the poem discuss how a flower lasts only for a short time. In the final two lines, the poem discusses how dawn is brief before turning into day.
The Vietcong set up checkpoints around the nation, so that would've definitely been a literal and figurative roadblock to any refugees. Hope this helped.