Queen Mary was born in 1867 and grew up when German nobles were everywhere, but especially the UK. Wars were infrequent and the aristocracy and nouveaux riches were living an international life style of grace and ease. War was dashing uniforms and heroic deeds. Britannia ruled the waves. She died in 1953 with the Savoias, Romanovs, Hohenzollerns, and Habsburgs deposed and other monarchies deposed in places like Romania. War, the classic role of royalty, had changed. A hydrogen bomb could destroy a metropolitan area and atomic bombs had destroyed cities. War was total. She had two sons who became King, but the first, Edward VIII, had to be edged aside to preserve the UK given his pro-Nazi leanings. Fortunately, his romantic choice of Wallis Simpson was untenable. The second son to become King, George VI, served his country, and the rest of the world, ably.
Her Wikipedia article is here.