Exodus 15–This part of the song is referring to the defeat of the army that opposed Israel. 2017

Exo 15:3
"The LORD is a warrior; The LORD is His name.

Exo 15:4
"Pharaoh's chariots and his army He has cast into the sea; And the choicest of his officers are drowned in the Red Sea.

Exo 15:5
"The deeps cover them; They went down into the depths like a stone.

This part of the song is sung referring to the defeat of the army that opposed Israel. That is why it will be sung again at Messiah's return also (Rev 14:12-20).