The main advantage of the North was in its economic and numerical power.  It also was helped by its navy and its relationship with other countries.  Its weaknesses had to do with strategy and with military talent.

To begin with what are probably the less important advantages, the North had a navy.  This was beneficial largely because it allowed the North to blockade the South.  This hurt the South economically.  The North was also in a better position internationally.  It was recognized by all the European countries while the South was not.  Therefore, there was no diplomatic pressure on the North.

The most important advantages the North had were economic and demographic.  The North had a much larger population.  Its advantage was even more clear-cut when we take into account the fact that much of the South’s population was enslaved.  The North had an even greater advantage in terms of industrial capacity.  It had almost all of the country’s manufacturing and a much better rail system.  This allowed it to produce and move everything it needed for the war.  The South could not do this and so its soldiers were often poorly equipped.

The North did not, however, have all the advantages.  First, it had the harder strategic task.  It had to invade and conquer the South whereas the South did not have to conquer the North.  Second, the South was much more of a military culture.  The South had most of the best officers from the antebellum US Army.  It also had soldiers who were much more used to riding horses and shooting and doing other things that were helpful militarily.

In the end, the North’s advantages, of course, outweighed its disadvantages.