Extending my vocabulary 1

The idea of this entry is, as the title suggests, extending my vocabulary. I chose 5 words to write sentences, I would like that you tell me, furthermore of the normal corrections, if I use the words like an English speaker would do.

Retreat: The armies of Napoleon retreated when they had arrived to Russia.
Ongoing: The errors when you're learning a new language are ongoing.
Rush: I must rush to speak better English
Depletion, Foe: The foes in the North Pole have suffered a depletion.

Thanks

Retreat: Your usage of retreat is correct, exactly as I would read it in my textbook.

Ongoing: This sentence is understandable, but in this case I believe there's a more suitable word for the occasion (I can't think of it at the moment however).

Rush: Rush is usually used in the sense of physical movement or motion. Such as "Jimmy rushed to the mailbox before his brother" or "The teacher rushed through the rest of the lesson before the bell rang" or even "Upon hearing the blow of the trumpet, the soldiers rushed into battle"(since you seem to be doing the war theme). Again while the sentence is understandable, it's not typically how an English speaker would use the word.

Depletion, foe: This is another perfect "textbook sounding" sentence with the exception being on depletion. It is used correct however, depletion means a loss of something. What exactly have the foes suffered a depletion of? A "depletion of supplies"? A "depletion of ammunition"?