Northern Advantages
One of the Union's most influential assets during the war was the sheer size of the Northern section. The Union soldiers that were in their prime fighting condition easily outnumbered the Confederates two to one. The difference here was that many Northerners tried to avoid fighting while most Southerners were willing and excited to fight. The North's next best aid was its greater industrial capacity. This meant it was easier for the North to manufacture and distribute better weapons. The North actually constructed almost all of the nation's firearms during the Civil War.
The U.S. navy remained in the hands of the Union, allowing them to institute a blockade on the South, cutting off their imports shortly after the war began. Because most of the fighting was done on Southern grounds, fewer Northern farms and crop fields were destroyed, allowing the Union to produce copious amounts of food that would sustain the armies. Possibly the biggest factor among the Union advantaged was the political structure of the North. The North's refusal to recognize the South as a separate nation impacted the views of other foreign nations. Because no foreign nation ever accepted the Confederacy, they never gained a political or military ally.