Be Strong is a Commandment!
Joshua (NASB) 1:
6 Be strong and courageous, for you shall give this people possession of the land which I swore to their fathers to give them. 7 Only be strong and very courageous; be careful to do according to all the law which Moses My servant commanded you; do not turn from it to the right or to the left, so that you may have success wherever you go. 8 This book of the law shall not depart from your mouth, but you shall meditate on it day and night, so that you may be careful to do according to all that is written in it; for then you will make your way prosperous, and then you will have success. 9 Have I not commanded you? Be strong and courageous! Do not tremble or be dismayed, for the LORD your God is with you wherever you go.
When the Israelites went out to war they would be addressed by a specially anointed Priest:
Deuteronomy (NASB) 20:
1 When you go out to battle against your enemies and see horses and chariots and people more numerous than you, do not be afraid of them; for the Lord your God, who brought you up from the land of Egypt, is with you. 2 When you are approaching the battle, the priest shall come near and speak to the people. 3 He shall say to them, 'Hear, O Israel, you are approaching the battle against your enemies today. Do not be fainthearted. Do not be afraid, or panic, or tremble before them, 4 for the LORD your God is the one who goes with you, to fight for you against your enemies, to save you.'
Maimonides in his Letter (in 1194 CE) to the Jews of Marseilles (France) said that the Jews in Second Temple times lost their independence to the Romans because they studied astrology and did not learn warfare.
This is why our kingdom was lost and our Temple was destroyed and why we were brought to this; for our fathers sinned and are no more because they found many books dealing with these themes of the star gazers, these things being the root of idolatry, as we have made clear in Laws Concerning Idolatry. They erred and were drawn after them, imagining them to be glorious science and to be of great utility. They did not busy themselves with the art of war or with the conquest of lands, but imagined that those studies would help them. Therefore the prophets called them 'fools and dolts' (Jeremiah 4:22). And truly fools they were, 'for they walked after confused things that do not profit' (I-Samuel 12:21, Jeremiah 2:8).