He could not marry a Catholic without losing his place in the line of succession. (I suspect that law will change relatively soon.) He COULD marry someone of any other religion, though it would probably have to be agreed that the children would be raised Anglican, particularly if by chance William did not already have children. In that eventuality, a child of Harry's would someday inherit the throne, and the monarch must be "in communion" with the Church of England.
Her nationality would be unimportant. Plenty of British royals have married non-Brits over the centuries. In fact, this used to be standard.
And one of the Queen's uncles married a Greek princess, while King George V's wife was German and HIS father's wife was Danish. There is no law against marrying a woman of color either. Some people might object if a child of Harry's were likely to inherit the throne (again that would be an issue if William had no children), but if the woman's character, education, and so forth were suitable, no one could legally prevent Harry from marrying her.
He cant marry a catholic without loosing his place in line for the throne. She would have to convert like Williams wife did. I don't think there would be much of a fuss if she was Canadian or from a country within the United Kingdom.
Im not sure that it would be a big deal if she was a French or American girl (although it would tick of the people) because the Monarchy is working on becoming more modern, so im sure they would allow french or American. Muslim however would probably be frowned upon.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.