Due to environmental concerns (very cold winters) humans are concentrated into a few large cities all around the world; smaller isolated settlements die out quickly. These cities would form the basis for nations, and each nation would usually be composed of only one city. A typical city would span many dozens of kilometers in radius and be self sufficient at least during the winter. These cities are, of course, built around rivers, lakes, and other high concentrations of important natural resources such as coal and forests for heating, iron for making tools and building and whatever other resources a city would need. Resources found farther out would have to have temporary camps and operate at a high output during the summer to make up for lost time during winter. If two city-states went to war against each other how would these wars play out? Massive multi-year sieges or would the enemy city be ignored altogether in favor of gobbling up the natural resources on their territory? Keep in mind that it is an unspoken rule of sorts that armies don't really fight each other during winter and instead focus on their own survival, I imagine this would make sieges difficult.
The period of technology I am focusing on would be late 19th century so there would be fairly modern artillery, bolt action rifles, early machine guns, etc.
Edit: It is not impossible to keep an army alive in the field during winter granted you have properly prepared and are well supplied, however don't even consider doing any kind of mobile warfare during the winter months.