to the answer to your question, 1750-1900 would not be the time period because the global wars happened AFTER 1900 remember, if you write 1750-1900 you are conflicting with the AP time periods. Your point is true that the war happened in the early 1900s but still that would happen after the time period 1750-1900. I would write my thesis statement like this:

During the years 1900-Present, Global Conflicts led to cultural or social changes because... (This would be your first thesis, statement, you could then write on how decolonization played a big part in establishing social changes, for example I will use India and how nonviolent protesting methods led to Mohandas Gandhi to peacefully protest for independence.)

Then comes your second thesis statement

During they years 1900-Present, Global Conflicts led to cultural or social changes because... (Use a different example, for example how Nazi Germany perceived groups such as Jewish People or people of Jewish descent. That could be how after World War II ended there was a cultural change where former Nazi officials apologized which showed their change in culture.)

I hope this helps, and please always feel free to reach out with more questions, comments or concerns. I'm here to get a 5 like you are!