How many people were sent to the army?

Moses took 1000 volunteers from each tribe.

Who did they fight?

The 1000 volunteers went with Pinchas the priest into battle against the Midianites.

What did they do after the battle with Midian?

Moses explained to the volunteers that if they touched a corpse or killed a person a purification process needed to be done. They would need to stay out of the camp for seven days. They also needed to immerse themselves and clothing in the mikvah.

What needed purification?
Every garment, every leather article, anything made of goat products and every wooden article must undergo such purification.

What was to be purified in fire?

Any gold, silver, copper, iron and tin that was used over fire (preparing food) must be brought over fire and purged and than purified again with sprinkling water.

If it wasn't used in fire (for making food) than it only has to be immersed in a mikvah.

What were the Jewish people required to do with the spoils won from the battle?

They were required to give a portion to the temple and Jewish people.

What was the request of the tribes Reuben, Gad & half of Manasseh?

They had captured many animals from the battle with Midianite. This group wanted to receive property that would be great grazing lands for these animals.

What did Moses reply?

You could receive the property on condition that you fight amongst your brothers till all of the land of Israel is conquered.The tribes agreed and went to fight for the lands on the side of the property across the Jordan from Jericho to the east.(Kingdom of Sichon, Og,Amorites,) including neighboring towns and villages along these borders.