According to the constitution, can the president decide on his/her own to bring the nation to war? why or why not?

ummm? is that supposed to be a question or a but this is a reason he won re-election

explanation:

"despite progress in the war, lincoln and most political pundits were convinced that he would lose his bid for reelection in 1864. the country was war weary and the democratic party's nominee, george mcclellan, was likely to negotiate a peace treaty with the confederacy if elected." - loc.gov

probally   a few days after there called