Serves as a leader: what he says, people will follow. Hence why it’s ludicrous for him to suggest ingesting disinfectant.
Commander in chief: Leads the strategy in military
Appoint ambassadors and chairs of cabinets and departments
Influences policy making by using veto power.
Can initiate trade.
Etc.
Some would argue the Soviet Union, while others say the US. It depends how you look at the question. There are certain things that prove it was the fault of one while some prove it was the fault of another.
At the beginning of the war, the Confederacy chose a more defensive strategy while the Union chose an offensive strategy. The defensive strategy of the Confederacy did not work against the strength of the Union army and the Union was able to make successful raids on Confederate territory. After a while, the Confederate army grew weaker and the Confederate President Jefferson Davis decided to switch to an offensive-defensive strategy that allowed Confederate territories to be protected but also allowed for strategic offensives or counteroffensives. The Confederacy didn't want to take Union territory, they just wanted to survive and defend it's right to secede. The strategies of the Confederacy could not work against the strength of the United States.
The Civil War revolutionized the concept of war. There were many technological advancements including the telegraph, railroads, trench warfare, hot air balloons, and ironclads. In my opinion, the most important of these was the telegraph. This invention allowed almost instant long-distance communication. If an armies telegraph wasn't working the outcome of the battle could dramatically change. The Union had a more reliable telegraph system than the Confederates, this could have been a major contribution to the North's victory. While railroads were invented in 1825, they were never really popular before the Civil War. During the Civil War, both North and South could move men, weapons, and food to there armies. The first balloon flight was in 1783. The Civil War was the first war to use air balloons for reconnaissance and artillery spotting. Trench Warfare was introduced during the war which prolonged fighting. Ironclads drastically advanced naval warfare because they were nearly impossible to sink.