The LORD Is My Banner
In stark contrast to their entry into Egypt in the time of their namesake ancestor Jacob (who the LORD renamed "Israel"; see Israel's Departure From Israel), the Israelites had passively surrendered themselves to the Egyptians, who by the time of the Exodus, the Israelites actually outnumbered (see The Israelite Egyptians). The Israelites had become a weak, sissy nation, unwilling to defend their personal and national sovereignty from aggression and oppression. Their liberation at the time of The Exodus happened, not because they stood up for themselves, but because the LORD had stood up for them.
"14:13 And Moses [see The Master's Messenger] said unto the people, Fear ye not, stand still, and see the salvation of the LORD, which he will show to you to day: for the Egyptians whom ye have seen to day, ye shall see them again no more for ever. 14:14 The LORD shall fight for you, and ye shall hold your peace." (Exodus 14:13-14 KJV)
Along with gradually restoring their personal dignity (e.g. by having to go out and work to gather manna for themselves; see Manna For The Children), the LORD also began to have them restore their national dignity by defending it themselves. The LORD would guarantee their victory against any enemy, no matter how big, if they obeyed His Law, but henceforth their battles were theirs to fight.
"17:8 Then came Amalek, and fought with Israel in Rephidim.
The "I'm a pansy no more" Israelites quickly proved themselves to be good soldiers, with moral support from Moses who stood as their ensign (see Ensigns).
"17:10 So Joshua did as Moses had said to him, and fought with Amalek: and Moses, Aaron, and Hur went up to the top of the hill.
Moses' upheld arms were viewed as their battle flag - the reason that he was given assistance to continue doing so.
"17:12 But Moses' hands were heavy; and they took a stone, and put it under him, and he sat thereon; and Aaron and Hur stayed up his hands, the one on the one side, and the other on the other side; and his hands were steady until the going down of the sun.
Moses was also given to be the national historian i.e. "Write this for a memorial in a book."
"17:14 And the LORD said unto Moses, Write this for a memorial in a book, and rehearse it in the ears of Joshua: for I will utterly put out the remembrance of Amalek from under heaven." (Exodus 17:14 KJV)
In Hebrew, "Jehovahnissi" means the LORD is my banner. The Israelites had a battle flag that day - the LORD.
"17:15 And Moses built an altar, and called the name of it Jehovahnissi: 17:16 For he said, Because the LORD hath sworn that the LORD will have war with Amalek from generation to generation." (Exodus 17:15-16 KJV)