The world changes ...
In the old days, political differences between countries were more often than not settled by war. That was only natural - under the ius ad bellum the right to go to war was an imperative of a monarch, which he was entitled to use as he saw fit and which was regarded as a normal tool of statecraft. And even then there were limits - possibly the first trial for waging aggressive war is that of Conradin von Hohenstaufen in 1268. One could do a lot of mischief, but not just as one pleased.
Over time the European practice of Cabinet Wars emerged - a type of war which affected Europe during the period of absolute monarchies, from the 1648 Peace of Westphalia to the 1789 French Revolution. These wars were characterised by small armies, noble officer corps, limited war goals, and frequently changing coalitions among the belligerents. There was a policy dispute over inheritance or a boundary, war was being declared (usually very politely), fought and settled. Wikipedia's entry sums it up rather well:
The Thirty Years' War, based on religious conflict, had been marked by wild plunders and marauding armies. Order was reestablished by the 1648 Treaty of Westphalia, which formulated the rules of international relations for the next centuries, in particular respective to the laws of war (jus ad bello and jus in bellum). During the Age of Enlightenment and under the direction of the "enlightened despots," wars became more regulated, although the civilian population was still a current victim of mercenaries. Such scenes as the 1572 St. Bartholomew's Day massacre became exceptional. Thus, Berlin was not plundered during the Seven Years' War of 1756-1762, despite having fallen into enemy hands not once but twice.
This state of affairs evolved during the era of the formation of nation states in the 19th century ito national wars with citizen particpation and conscript armies. The American Civil War and battles like Solferino (the carnage of which led to Henry Dunant initiating the Geneva Conventions and the Red Cross) offered glimpses into what technological advances in war were to bring in the next century.