Lowering unit morale during war?

I have declared war to a country some time ago, and my troops have reached it nearly 1 hour ago and the fight started. I had my naval forces there as well, in order to help me defeat the enemy easier.

But there's a thing I don't understand. How is the morale of my troops and my enemy's getting lowered?

In case it helps, my terrain troops had a force of 12.0(three infantry troops and 1 armored car), now dropped to 10.8 and 83% morale, and my enemy's terrain has 53% morale(dropped from 70%) and a current force of 2.9(2 infantry troops)