Bible students do not know who wrote this psalm. And they do not know when they wrote it. Perhaps it was before a war, or after a war which God’s people, the *Jews, won. The war that Christians fight is not with guns and knives. It is a war with *prayer and love instead of guns and knives. Read some of the verses about Christian war in Something to do. You say a *prayer when you speak to God.
Some Bible students think that Psalm 149 is not only about a war that the *Jews fought, or even a Christian war. It also makes us look at when Jesus will come back to the earth. Bible students call this "eschatology". It is the study of what happens at the end of the world. That is when God will have vengeance on his enemies. "Vengeance" means "to hurt (or punish) people because they have hurt other people". God has often used some people to *punish other, bad, people. But at the end of time, this is what the Bible tells us. God will use his people to give him help! Read Matthew 19:28 at the top of this psalm. Here it is the 12 *disciples who will give the help. Paul, in 1 Corinthians 6:2 says, "The *saints will *judge the world". The word "*judge" means "decide if people are good or bad". If they are bad, it also means "decide how to *punish them". The word "*saints" is how the early Church described all Christians.
But some translations of Psalm 149 also use the word "*saints". They use it in verses 1, 5 and 9 where we have "his people". The *Jews spoke the Hebrew language. The Hebrew word for "his people" or "*saints" is "chesidim". It really means "people that have accepted God’s kind love".