When Amalek came to fight with the Jewish people, Moshe sent Yehoshua to fight the war while he went on the mountain to pray, Hashem made his hands get heavy because he sent some else to fight the war. Although Moshe had good intentions being that he was 80 years old he figured he should send young men to fight while he prayed, Hashem was not happy because if someone comes to attack another Jew you shouldn’t make any calculations you should just go out and help. לקו״ש כא