In Surah Ali Imran 3:123 it says:

And already had Allah given you victory at [the battle of] Badr while you were few in number. Then fear Allah ; perhaps you will be grateful.

The word by word translation from Quran.com of the phrase in bold is:

So fear - Allah SWT - So that you may - (be) grateful

What is the meaning of this phrase I don't understand. What is the relation between fear and being grateful.

Being grateful is being grateful for something of the past. But fear is, as far as I know (but CMIIW), is fear to something that may happen in the future. So how come fearing something makes somebody grateful? What's the relation? Can anyone explain on this.

Regards