I like it! I spent an enjoyable 10 minutes clicking around and checking out what battles took place at various places I have lived in the US and Europe. Is there any way of adding country/US State boundaries for the more geographically-challenged among us? I can usually find my way around using the coastline and following rivers, but I get lost in the middle of large continents occasionally.
For Europe you could use easy trick - 4 centers (Germany, France, Italy, UK) divided by large mountains or large water. Only exception is west Germany land border.
It become early obvious, battle on mountains or coast battle are ineffective.
So, most European battles happen on flat land.
Is the increased density in Western Europe caused by unintended bias on Wikipedia (more likely to be documented) or that those places were more war hungry?
Bias, absolutely. I just know, Russia was at war nearly all ~400 years it exist, and with that "liberation" wars against all surrounding peoples, over last 200 years achieve grow of territory about 2x.
So, definitely, Russia should have at least comparable to Europe number of battles on their territory.