For the little that I know Europe dominated scientific and mathematical research at the start of the 20th century. Then due to the wars and improvement in other countries this leading role was lost.
I would like to know how and which gradually changes produced the shift. Can someone suggest sources to read more about this topic? (summaries are welcomed too)
Many scientists fled Europe in the 30s, mostly to the USA. But for those who stayed many problems arose during the following years.
It can't be emphasized enough how different the impact of WW2 was for the United States versus Continental Europe. For the years of the war fundamental research in many scientifically leading nations of Europe was all but dispended. In Germany for example one had to fear for his status of "UK" ("Unabkömmlich") ...which means the person was deferred from military service as long as their activity was "kriegswichtig" (essential for the war effort). Many scientists struggled to find ways to preserve their "UK"-status. More practical research with quick results was one way of doing that, neglecting theoretical research.
And don't forget that the Nazis were forbidding Einsteinian physics and other Jewish accomplishments, narrowing and crippling their own science departments.
In America things were different, the pressure on the universities was lower, in comparison. Far lower percentages of the male population had to serve for extended time periods.
It were basically 5-10 years lost through WW2 and its aftermath.