RASHI: וימסרו SO THERE WERE HANDED OVER … [A
THOUSAND OF EVERY TRIBE] — This serves to tell you in what high esteem the shepherds of Israel meaning the Leaders-
were really held — how dear they were to Israel.  

Up until they had heard of his approaching
death what does it state? That Moses felt constrained to exclaim about them, “They are almost ready to stone me”! (Exodus 17:14). But when they heard that Moses’ death was associated with the execution of vengeance upon Midian (cf. v. 2), they refused
to go to war until they were “handed over” against their own will (Sifrei Bamidbar 157:3). 

 Rabbeynu Bachya  

ו 

וימסרו מאלפי ישראל, “they were
handed over from amongst the thousands of Israelites, etc.”  

 Rabbeynu Bachya: Our sages in Sifri Mattot 157 comment on this
that these men had not volunteered but had been drafted against their will due to their love of Moses; they knew that Moses’ death would result from their waging this war and they did not want to contribute to Moses’ death