As said above, you mostly cannot defeat a large nation within a few years. The EU3 peace mechanic demands you to wage multiple wars to get the enemy provinces step-by-step. This is very different from Viki or HoI, but once you get used to it, it is actually fun, because a defeated enemy has in fact a chance to get his revenge.
The game lasts for 400 years, so 10 years is a very, very short timeframe, basicly I have sometimes fought wars for 15 years (bringing both me and my enemy to the edge of destruction), so - don't be too hasty. EU3 is a game of patience. It is not about building up a huge army and steamroll the whole world, but rather you have to make small steps and increase the size of your country.
For example: As Spain, you defeat Granada and take 2 of their 3 provinces, in a second war 5 years later you annex them. You set up a friendly relationship with France and England to keep your back safe. Then you go for Aragon and take some provinces. The next target is North Africa, you try to grab some provinces from Morocco. After a 5 year peace, you beat Aragon again. Meanwhile you colonize the African westcoast. In a third war, you annex Aragon and wait 50 years to get your cores to form Spain. As soon as possible, you either go for Portugal or sail to the new world - or do both. You colonize the Carribians and desperatly try to prevent the French from attacking you. If they do, you fight a defensive war and try to save your hide. Then you colonize north and south america, once you are eye-to-eye with France, you can start attacking them... And when this happenes, it is probably already around 1550 or something...
Or, for example if you start in 1399 and plan to destroy France, it will take you about 50-200 years, depending on your skill level and the AI reactions. If you are lucky and the AI makes a blunder, you can take out France quite early (meaning: weaken them so much that the next war won't be a problem), but most of the times you will have to expand your economy stand a chance.
And the larger your foe becomes, the more difficult it is to crush him and make his realm collapse.
So basicly: You can defeat Napoleon and sign a white peace (or make them releas some minor countries) in your timeframe, but "defeating" France in terms of getting major parts of it's posessions - no way. My guess is you would require at least 50 years for this. At least.
Patience is the key. Slow and steady wins the race!
