I agree with Dan. You can either talk about WHY the president came to the decision, or you can talk about HOW he came to the decision. "Reason by which" confuses these two issues. WHY: "the reason why he came to this decision" HOW: "the reasoning by which he came to this decision" or, better, "the reasoning that led him to this decision"

This sentence has poor grammar in many ways. I would change the start to: "I think it's important to understand, that's precisely..." Next, if we're saying "the reason", it would be better as "the reason why..." If we're saying "by which", it would be better as "the reasoning by which..." We can also say "the logic with which..." or "the rationale via which..." or "the end goal for which..." Also, "the president" doesn't need capitalising. The capitalisation is only needed when you're naming him, such as "the reason why President Bush..."

Yes

No, "by which" and "for which" are not the same and can't be used interchangeably in your sentence. * "By which" refers to the method or means through which something happens. * "For which" refers to the reason or purpose for something. In your sentence, "by which" implies the process or means the President used to come to the decision. If you use "for which," it would shift the focus to the reason for the decision, which changes the meaning. In this context, "by which" is more appropriate.