Once again, Millay is pulling out some tried and true tricks here. The history of poetry is littered with apostrophes—moments when the speaker of a poem addresses an audience directly. Milton called upon the muses. Shakespeare called upon the muses. Coleridge directed his poetry at the sun. Millay does something a little different, though: instead of addressing some inspirational figure or a holy guiding principal, she directs her speaker's apostrophe in two directions: to both her friends and her foes.
When the speaker cries out in line 3, "But ah! my foes and oh my friends," she's drawing a pretty clear line in the sand. Either you're with her (her friend) or you're against her. And since we usually want to be on the good side of our literary speakers, chances are we're going to assume that we're her friends. That puts us in a pretty interesting position in relationship to her enemies. Are we supposed to hate them, too? You could think of this as an early version of Mean Girls—after all, the easiest way to get into a cool crowd is to point out someone else that isn't cool, right? By splitting the apostrophe, however, Millay leaves us slightly unsure of where we stand. Either way, though, we're clearly supposed to be dazzled.