How can minister of defense...

[mastrude]

control other players' armies. I'm trying to assign them to the battlefield and dont' see how. Or do players have to assign them to the field first?

 

[eewallace]

The other players have to send them into battle. As Defense Minister, you can set them to heroic, normal, fortify or retreat.

 

[mastrude]

The other players have to send them into battle. As Defense Minister, you can set them to heroic, normal, fortify or retreat.

I have tested this and it appears that you are exactly right.