In "A Poison Tree," what is the literal meaning of "And into my garden stole"? How did the persona's enemy enter his garden?

In "A Poison Tree," this line literally means that the enemy snuck into the speaker's garden. The word "stole," for example, means to sneak or creep into a place. However, there is also a deeper meaning here: by using the word "stole," a word suggestive of stealing or theft, the speaker implies that the enemy entered without his permission. So, in one respect, the enemy is like a thief or a criminal.

In the next...

In "A Poison Tree," this line literally means that the enemy snuck into the speaker's garden. The word "stole," for example, means to sneak or creep into a place. However, there is also a deeper meaning here: by using the word "stole," a word suggestive of stealing or theft, the speaker implies that the enemy entered without his permission. So, in one respect, the enemy is like a thief or a criminal.

In the next line, the reader learns how the enemy was able to sneak into the speaker's garden:

When the night have veiled the pole.

So, the enemy entered the garden at night, using the cover of darkness as a sort of protection against being detected by the speaker. In fact, it was so dark that night that the "pole," a reference to the North Star, could not be seen. Thus, the night sky helped to camouflage the enemy.