Exodus 17:8-16  The Israelites are attacked by Amalek.
This is their first battle as freed slaves.
The first time they have to put together an army.
The first time they have to take up swords... how did they even have swords?  It had to be pretty frightening .
But it seems like they were ready for action.  And amazingly, there was no grumbling!
Joshua put together the army and led them in battle.
Moses went up to the top of a hill and interceded.
This story has been told in so often.  When Moses has his arms lifted, they are winning.  When he puts them down, they are losing.  A man can only hold his arms up so long on his own... so Aaron and Hur come along side and help Moses by holding his arms up for him.
The Lord brings victory against Amalek.
And He never forgets what they did... choosing to "utterly blot out the memory of Amalek from under  heaven."

Deuteronomy 25: 17-18 " Remember what Amalek did to you along the way when you came out of Egypt, how he met you along the way and attacked among you all the stragglers at your rear when you were faint and weary; and he did not fear God."  When Israel entered the Promised land, they were to finish Amalek off.  "..you must not forget"  He reminds them.

In I Samuel 15 King Saul's first assignment is this.  "Thus says the Lord of hosts, I will punish Amelek for what he did to Israel, how he set himself against him on the way while he was coming up from Egypt. Now go and strike Amelek and utterly destroy all that he has and do not spare him; but put to death both man and woman, child and infant, ox and sheep, camel and donkey."
Saul almost does the job, but falls short by sparing Agag the king and the best of the animals.
(Which displeases God and causes Saul to lose his blessings from God and his position as king. )

Amelek was a grandson of Esau.  A great-grandson of Abraham.  When all the nations heard what the Lord had done in bringing out Israel from Egypt, they were afraid... except for Amalek.  The passage in Deuteronomy shows why God was so angry with them.  They attacked the stragglers. Those who were faint and weary... the elderly, the weak, the sick, the lame, the ones that couldn't keep up.  These distant relatives had no mercy for those who had so little.   The nation of Amalek was arrogant and selfish, greedy and cowardly... all at the same time.   The Lord would no longer tolerate their sins.

Father in heaven,  You are the Righteous Judge and You will bring judgement upon all who refuse to fear You or obey You.   You are the Helper and Protector of those who are weak and weary.  Thank You for hearing our prayers, just as You heard the prayers of Moses.   We know that we are also in a battle,  though our battle is "not against flesh and blood, but against the rulers, against the powers , against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places." (Eph.6:12)  We pray for You, the Lord of Hosts, to fight for us.  To deliver us from evil.  To lead us in righteous paths. Help us to trust You to remove the bullies that attack the most vulnerable among us.  We take up our armor - Truth, Righteousness,  Salvation, Peace, Faith, and the Word and we pray.  We stand firm and we persevere in prayer.  And when are arms grow heavy, we come along side one another and hold each other up. So that Your Kingdom will come and Your will is done.  So that Your Name will be exalted and honored among all the nations.
Amen.