Lincoln was originally elected with the intention of preserving the union instead of abolishing slavery. He later shifted that cause to that. Lincoln was a Republican from Illinois. Lincoln was a very open-minded leader and this led to a great leadership.
One of the major military leaders was Ulysses S. Grant. The first major siege under Grant was the siege of Vicksburg. Grant led the first capture of Jackson and pushed the confederate troops back in to Vicksburg. This victory bolstered Grant's reputation. Then General Robert E. Lee surrendered his confederate army to Grant at Appomattox. Grant is responsible for these two wins and many more because he was an intelligent man when it came to military strategies and he knew how to lead.
Grant "entrenched" the confederate army in Vicksburg, the Union surrounded them and they surrendered after Grant launched a couple of missile assaults and continuously terrorized Vicksburg for the next 6 weeks. This siege on Vicksburg was one that highly boosted his reputation because it was pulled off very well.
Since the North was far more industrialized with mills, factories, and weapons they had a massive upper hand in the war. Modern weapons were beginning to form, they were being created, and technological way the North was way ahead of the south. The North held 90% of the nation's factories so they did have a massive advantage with that. They supplied the Union troops with the right weapons.
At this point in time, America has had its independence for quite some time where the confederate was a newly found nation. The union had much more money than the south and since they did they could supply the right amount of weaponry, food, clothing, etc. unlike the North the South had a lot less money and this would have been a problem. It was like the American Revolution all over again because one side was well conditioned and the other was decently conditioned.
In the Union there was a population of 22 million. This was a severe advantage since they could supply troops and raise the army and have many others support the war effort. The Confederates did not have a massive population like that so their troops were not in large quantities. This can cause the loss of any war regardless of the cause.