Use the War command and select a general to go into battle.
Assign the maximum number of men to his unit.
When they asked for confirmation for going to war select the ''No''option.
Remove the general from the list of commanders and reassign him to the post.
Return to the troop assignment screen.
The number of men available should be multiply by the number that was assigned earlier.
Repeat this process to increase the number of men, but don't go back to the main menu.