With a POD after 1901, have all the great powers of the world exist as liberal democracies by the present day. These may or may not be the current great powers - for instance, the list could include Austria-Hungary or something - but each country considered a great power at that time must have a fully-democratic government with wholly free-and-fair elections, minimal vote rigging and freedom of speech/assembly/etc. Monarchies can qualify, but any monarchial head of state must exist as a virtually-poweless figurehead.