You could say that, but the level of involvement in the "advisory period" of the Vietnam War must have been matched in various Caribbean ventures in the early 20th Century. I suppose that you might as well count Lincoln, for secession preceded his inauguration.
Secession wasn't the official start of the war, I would say it was Sumter, which happened shortly after Lincoln took office.
I was going to mention Lincoln, but he was in office during Sumter and when he died, Lee had already surrendered the confederate army. It's fairly fuzzy too because there were battles after the surrender.
