Maybe the root of your question aim to the surge of the concept of nationalism, and later on self-determination and the United Nations.
Nationalism surges as a concept after French Revolution, and later on in 1848, it means that countries should be composed by a single nation of people. While self-determination surges with United Nations, where its idea is that people should select how they want to chose their sovereignty.
In the past, a territory was an asset of a king or a country, so they had the right to yield a territory as an exchange of peace. But with the new concept of self-determination, it was no longer acceptable to give oor take territory without consulting the people that lives there.
With the current concept of self-determination, people of Alsace and Lorraine should have been consulted whether they wanted to be germans of frenchs.
Based on these concepts, is no longer easy to set peace terms, because any exchange of territory involves the right of the people of that territory to revolt against their new ruler (western bankfor example, the West Bank territories controlled by Israel). Or the contrary, a nation of people can demand their own country agaist their current ruler (Kosovo).
Besides, United Nations concept is to prevent wars between countries, not to solve problems of self-determination. Hence, war is no longer acceptable to solve territorial disputes like it was in the past.