Various comments have been made on this verse, but one thing is clear enough; that it came down after the battle of OHUD, and it is related to that war.
The incident of OHUD carries great lessons that, God's help will come if we believe in him, and be obedient to his commands and have discipline and patience. If we fail; His Mercy is always open to us, but we should know that it is also open to our enemy, and all his creatures.