What the latest time a petty noble could have seized a major throne in Western Europe

The Karadjordjevic were definitely not petty nobles. Peter I was the son of Alexander Karadjordjevic who had been elected Prince of Serbia back in the 19th century and was later deposed by Parliament. Not to mention the original Petrovic who is actually credited to be the founder of modern Serbia (even though he was a peasant with no royal blood, but then again so was Milos Obrenovic). So what the Karadjordjevics actually did was restore themselves to the throne of Serbia.

Fair enough.
 
Germany post Great War?

Perhaps an alternate far right nationalist might try to get himself crowned king in the 1930's as a way to consolidate power. It would be tough, but showing some distant connection to the Kaiser might help. (Just make sure that the ink is dry...)
 
Top