"I think that it should require only the public's interest and soldiers willingness."
The public isn't interested enough in world events and soldiers never want to go to war. US Soldiers will go to war because the president orders them. US Soldiers want to defend the security of the US. US Soldiers have strong sense of duty and must have strong faith in the ability of their superiors to make decisions which are best for an entire miltary action. But the main strength of our miltary is that soldiers trust their fellow soldiers.

"If US armed forces and the public believes a cause is worth fighting for on moral grounds I think we should regardless of the presence of national interest."
This is the job of the President. He/she must "know" what is in the national interest and must "know" the capability of our miltary. National interest is the interest of the US public. The reason you should vote for a president is that you are confident that this person will represent your interests, and will perform the duties of the office.