It was probably a vole (field mouse). I have had one pull two different 4" geraniums down a hole... Dang it! All it left was 2 leaves and a bloom.. Then it came back for those the next day. The little buggers nipped off alot of iris also..
I have had a problem with these critters alot over the winter and spring.. I have had some luck dropping rat poison pellets down thier holes and along their paths. I make sure to get the pellets down in the hole and conceal the ones on the paths to make sure no others will be harmed. animal or human.

Looks like a bunny to me. The rabbit in my front yard makes the most amazing sharp diagonal cuts like that...

I vote bunny too. Especially since you have the cut off stem. I can't tell you how many times I've found that diagonal cut and the stem just laying next to the plant. I decided to try Milorganite this year and it does help.

I agree - bunny. I love peace too but have you thought about getting a cat?