Another reason the European model of diplomacy and appeasement could work during the period between the Treaty of Westphalia and the First World War is that there were only a few European Great Powers and a great number of smaller powers who were arrayed in endlessly shifting coalitions such that no power ever became overwhelmingly powerful on the Continent. At least from the time of Elizabeth I, maintaining the Balance of Power was an explicit foreign policy of England.
In such a world, no war is really l'outrance and wars are not wars between peoples, but states. Of course, things began to change with the Napoleonic Wars, but even Napoleon was defeated by a Grand Coalition, and the Balance of Power restored at the Congress of Vienna in 1815. While liberty was trying to grow in Europe (think 1848), the European wars in the 19th century (if you could call them that) were all really fought pretty much on the old model: Crimean War, Prusso-Danish War, Austro-Prussian War and Franco-Prussian War. (Even though the primary underlying issue in all of them was the unification of the Germanies. Of course the other lesser known wars of the 19th c. in Europe also concerned the unification of Italy).