Adam Clarke on Exodus 17:15

Jehovah-nissi - Jehovah is my ensign or banner. The hands and rod of Moses were held up as soldiers are wont to hold up their standards in the time of battle; and as these standards bear the arms of the country, the soldiers are said to fight under that banner, i.e., under the direction and in the defense of that government. Thus the Israelites fought under the direction of God, and in the defense of his truth; and therefore the name of Jehovah became the armorial bearing of the whole congregation. By his direction they fought, and in his name and strength they conquered; each one feeling himself, not his own, but the Lord's soldier.