Loss of National Morale

[aprezto]

I just recently had an alarming loss of national morale and I can't seem to find the reason.

I was playing as Prussia and was at war with France. I just lost a battle where I lost 20K troops to his 12K. His troops were not in my capital, and I had no other battle. I went to war as part of a tri-partite treaty.

After the battle I had been reduced from ~45 NM to -450!!! What is likely to have caused such a loss?

I am playing advanced econ - it took me 30 turns or so to get the NM to 45 as it was (I struggled to get enough luxuries if I wanted to produce anything other than them).

[Anthropoid]

Hmm, just guessing here . . . were you an Empire and then did you for some reason lose Empire status?
 
Do you have any other treaties standing?
 
I suppose casualties might be figured in to Nat Mor calculations. Probably one of the experts will respond here in a little bit.

[Russian Guard]

Check the battle - you can lose up to -500 NML for a large battle loss, in addition to casualties NML loss as well.