When Merkel decided to welcome one million refugees on german soil, it was about domestic politics?
No, that was about personal revenge for the press eviscerating her and calling her the "Ice Queen"[1] after an interview segment was shown on TV in which Merkel can be seen rejecting a refugee girl's teary-eyed plead to bend the asylum law solely for her.
[1] https://pbs.twimg.com/media/CKBa2VvUMAE1Ymv?format=jpg&name=...
[2] https://www.aljazeera.com/opinions/2015/7/29/a-refugees-hope...
Basically any diplomatic actions that on their face appear to be overreactions, or intense provocations, are better understood as communicating something to a domestic audience in a context that rewards it.
Classic example is "wolf warrior diplomacy", where tweets from Chinese officials can seem to indicate that the PLA is about to deploy tanks when in reality it's just a diplomat trying to look as nationalistic as they can for their bosses' approval.
Or when France opposed the Iraq War in 2003, the "Freedom Fries" nonsense in America was really more about the general patriotic hysteria of the moment rather than a sign that there was any enduring hatred for France.