After WW1 the Allied Powers set harsh terms for the Germans under the treaty of Versaille. They wanted the Germans to pay back all the damages they caused during the war. This lead to hostility by the Germans since they couldn’t pay back their debts. We all know how that worked out.
When the US levelled Germany after WW2 they didn’t later boot all the Germans out of Germany.. They still had a homeland to go to. After the war the Americans even pumped their own money into Germany to help them rebuild. The next 80 years (and counting) were the most prosperous and peaceful in European history.
In game theory when everyone always purely seeks their self-interest at the expense of the other it eventually ends up in a worse outcome for everyone. When both parties attempt to be more forgiving and cooperate with one another the outcome is better for both,.