“To enable him to escape” vs. “to enable him escape”

"To" isn't a preposition here, it's part of the verb. Sentences like this generally use the full infinitive (or to- infinitive) to introduce the subordinate clause. Like I just did there.

She enabled him to escape capture.
She helped him to choose a new wardrobe.
She told him to go away.

Help turns out to be able to use a bare infinitive instead:

She helped him choose a new wardrobe.

but not

She enabled him escape capture.

With some verbs, you must use the bare infinitive. Mostly these are verbs concerning perception or permission; unfortunately you just have to know which ones are which.

She saw him run away.
She let him go.

but

She allowed him to go.
She observed him to run away. (This particular one is very unusual, and I wouldn't use it if I didn't have to!)