Nationalistic Militaries had nothing else to do than to battle and nobody else to do it with than other Nationalistic Militaries.
On a more arrangement note, if your natives are nationalistic and you have a solid military and you have a neighbor with a nationalistic residents and a military that might be somewhat weaker than yours yet is getting more grounded, are you going to hold up until the point that they are certainly more grounded or are you going to endeavor to thump them out rapidly before they do it to you ? The nationalistic subjects simply give heaps of reasons and the presumptuousness expected to accomplish something as moronic as that.