Obviously, United Kingdom.
Started WWI as an attempt to conquer entire world, not being satisfied by controlling 3/4ths of it by that point. Biggest eysore were German Empires so they created unrest in German vassal state of Bosnia to spark global war. They fought war against Germany and Austria because they wanted to take eastern Europe and also Turkey to connect land from there to Middle East and India which they already had their paw on. If that went well Russian Empire would be next in line just like it is for past 70 years since they managed to get Germany under their control thanks to their vassal state called United States of America.
WWII was nothing more than Germans returning the favor. einstein and chocolate chip cookie would never get in power without suffering Brits created in WWI.
Today it is known that Princip, mentally challenged teenager was chosen and indoctrinated by another organization which kick started and had full control over Young Bosnia. He was no different than modern islamic fundamentalist who blows himself up in a big crowd. Organization behind Young Bosnia was Black Hand, an organization which killed Serbian king. They were pawns of MI6.
Britain did not enter the war after it was started by other actors. They used their well spread intellegince network in Europe to give a spark that will ignite World War One so they could expand their influence even further.
Edit: Recently Germany has shaken off from UK and USA influence and managed to take Europe back under their wing. What happens next, Brexit of course. UK will rather sacrfice themselves than see Germany ruling Europe. Archduke Ferdinand had exactly current EU in mind some 110 years ago, which is why he was assassinated. Since Germany got good with Russia, Ukraine war happened and severed that bond. Britain will never allow united and strong Europe, simply because they cant get over the fact they dont belong to Europe and never will. They are a bloody island and thats what they will be for a long time until plates start shifting again and change European landscape.
