Can you begin a sentence with for as a conjunction and not as a preposition?

No. For this to occur, the Word 'for' must be a preposition.

Example: "For the final battle, the emperor chose his strongest warriors."

When the Word "for" means "because" it has to follow the independent clause.

"We must surrender, for we have nowhere to hide."

Compare this to using "as" in exactly the same context, where it can come first.

"As we have nowhere to hide, we must surrender."

This appears to be a quirk of using "for" as a conjunction.