Who is the first president of US

What would lead a historian to consider two events to have a relationship of correlation rather than causation?

Who had the idea that there should be 3 branches of government

1. how did manifest destiny spur nation building in the u.s.? 2. how did the idea of “the nation” begin to change? 3. how were the unifications of germany and italy similar? 4. what problems did the austro-hungarian empire have? 5. what domestic issues did england and france have?