Getting the relations to positive will take about 7 years. If you force a Personal Union then you junior partner will start the Personal Union with negative relations towards you because you just fought in a war against each other. Because every time your king dies and your heir takes the throne the game will check if you have a positive relation with your junior partner. Once you get a Personal Union you will have to improve you relation with your new junior partner. With this method you can take over a lot of big nations. If you win the war you can force them to be in a personal union with them. Next you attack them with the casus belli you just got from claiming the throne. This can be done in the diplomacy window. Instead you wait for the nation you want to take to have no heir and you claim their throne. But like I said, you have to get extremely lucky and you want world conquest so have no time to wait for that.

If you wait and the king of the nation you have a Personal Union with dies without a heir and you have high prestige then you can inherit there nation in a Personal Union. Next you can either wait to get extremely lucky or force a Personal Union. The first step is to get a royal marriage with the nation you want a Personal Union with. You also get a certain amount of income, money, from your Personal Union. The junior partner will always join your battle's and fight for you. The country leading the union the senior partner gains the throne and dominates the country who lost their monarch, the junior partner. Personal Unions are a very powerful way to expand your nation. Just like a lot of people say it's not possible for England to win the Hundred Year War that starts at And this is probably the best way to start your World Conquest. Ok, so a lot people online say it's not possible.

Eu4 world conquest guide