star wars - Why did Palpatine decide to take over the Republic?

I remember someone told me (or maybe I read it somewhere) that there was a big incoming alien invasion to the Republic.

Palpatine learned about it and realized that a peaceful Republic could not hold against it so he decided to perform a coup on the Republic so as to have a "strong" leadership with a huge military able to defend from alien invasion.

Is this correct, and why Palpatine decided to go for the dark side?

Was he a normal Jedi first?