Dumbledore was an excellent "discoverer." He found many useful talents among his students, such as Newt Scamander and Hagrid. Both had been widely regarded as oddballs at school, and Hagrid had even been expelled.
Strictly speaking, Grindelwald had not caused any chaos in New York. It was the Obscurial, Credence, who had done it. He had merely manipulated Credence from behind the scenes and impersonated an Auror.
He played the part very well. At the very least, no one suspected that the Auror was not the real one.
Jurisdiction could be divided into jurisdiction over persons, matters, and property. Since Grindelwald had not killed anyone in America, he could not simply be hanged after capture like a pirate. MACUSA could only detain him, then choose either to hand Grindelwald over to Europe or try him themselves. After World War II, universal jurisdiction was extended to other international crimes as well, such as apprehending Nazis.
But all that applied after Grindelwald had been captured. The authority to arrest was another matter entirely.
Georgiana took out her British passport. According to the Magna Carta of 1215, citizens should have the freedom to enter or leave the kingdom, yet Scamander had entered France by illegal means.
First, the European Union had not yet been established, so he could not travel freely between member states.
Second, Grindelwald had indeed turned some people in the French Ministry of Magic, leaving it short-staffed, but that did not mean every Briton could meddle in their internal affairs.
Scamander was not on France's list of approved entrants; he was on Britain's travel ban list. Unless he joined the Ministry of Magic, which could then submit his name to France, and the French stamped the list, he would be able to operate like the other British Aurors.
Scamander had