The major Northern strength that led to victory for the Union was the North’s huge advantage in population and industrial power. The corresponding Confederate weakness was a major factor in its defeat. Going into the Civil War, the North had a tremendous advantage in population and industry. As the link below tells us, the North manufactured 90 percent of the boots and shoes produced in the country. It produced 93 percent of the country’s pig iron and 97 percent of its guns. The North was home to almost five times as many free people as the South. All of these things were huge advantages for the North. These factors gave it more men to throw into the war and more weapons, boots, and other things with which to equip them.
There were some other Northern strengths that mattered to some degree. The North had a navy while the South really did not. This allowed the North to blockade the South, helping the North to win the war. A Southern weakness that helped cause the Confederacy to lose was the fact that it was a confederacy. The Southern states did not want to allow their national government to have much control over them. This led them to do things like resisting putting up the money the national government needed for the war.
The most important factors, though, were population and industrial capacity.