And into my garden stole When the night had veiled the pole; In the morning glad I see My foe outstretched beneath the tree.
Aha! The enemy has seen this anger-apple in the speaker's garden. So, it's safe to say that's how he knows it is the speaker's.
That doesn't stop the enemy from trying to steal it, though. After he has seen the apple, the "foe" sneaks into the speaker's garden at night.
The word "stole" is a past tense of the verb "steal," which in this context means something like "sneak in secretly." This word also suggests "steal" (like a thief steals). It seems that the speaker is blaming his foe, or calling him a thief.
This happens when it's super-dark out. In the phrase "night had veiled the pole," pole refers to the top of the earth, as in the "north pole," but it can also mean the pole star, also known as the North star, also known as Polaris. It's an important star for navigation, since it's bright and it stays pretty much fixed in the sky. Tonight, though, the night has "veiled" it, covered it up. This star, used in navigating folks safely through danger, is not visible. Uh-oh!
To suggest that the night (an abstract time) could actually cover up the star (like a person might) is to use personification.
Apparently, at some point in the super-dark night, the enemy eats the apple, which ends up killing him or making him fall asleep. It's not clear which, although the speaker is glad to see him laid out in the garden. We're going to go with death for the enemy here, since the speaker would likely not be too happy if his enemy both ate his apple and used his garden like a cheap hotel.
Still, the word "glad" is a bit ambiguous here (it could have more than one meaning). "Glad" could refer to the morning, as in "the morning is glad," or it can refer to the speaker's feelings when he sees his "foe" lying "beneath the tree."
Either way, it seems like bad times for the enemy, good times for the speaker. Or is it?