I have to agree with the main points of the previous posts in that the North's superiority of numbers proved to be their strongest asset. Militarily, the South held the advantage at the beginning of the war. Their cavalry was far superior for much of the war, and Southern leadership at the upper ranks was also at a higher level. However, as the war dragged on--and with it mounting casualties--it became impossible for the South to replenish their ranks. In the North, the population was far larger in numbers, and the nation continued to grow through immigration during the war years. When the call came to add Negro troops, it further enhanced the Union's military numbers.
The Federal navy was far superior to the Confederacy's at the beginning of the war, and it only became stronger as the war progressed. Eventually, the Union cavalry became an even match for their Southern counterparts. When Lincoln finally discovered great commanders (Grant, Sherman, Sheridan, Thomas) at the highest levels, it was only a matter of time for their numerical superiority in all branches to overwhelm the Confederate forces.
Northern strengths and Southern weaknesses in the War between The States:
Northern strengths include a large industrial base, a larger population from which to draw both soldiers and industrial workers, soldiers recruited from Europe with promises of land (the Pope issued a letter opposing this), black soldiers (many volunteers and many impressed), a central authority (Abraham Lincoln) that could and did imprison thousands of citizens who opposed the war, a war leader (Abraham Lincoln) who was good at working with other politicians and who finally became able to turn the fighting of the war over to his generals.
Southern weaknesses include a small industrial base, a smaller population, a resistance to using blacks as soldiers (resistance on the part of politicians; generals enrolled tens of thousands of black soldiers, but not the hundreds of thousands that the North enrolled and that the South could have enrolled but for political opposition), a weaker central authority so that some state governors (notably Ga., and N.C.) did less than they could have done to cooperate with the national leaders of the war effort, a war leader (Jefferson Davis) who was not good at working with other politicians and never became able to turn the military side of the war over to military men.
All of the previous posts were very much accurate. I would add my own opinion to the mixture by suggesting that the mentalities of both sides helped allow for victory and set the stage for defeat. The North understood that they had a greater chance of winning if they could extend the war, use their industry to end up defeating the South through attrition, and allow their superior numbers in manpower to take over. This mentality allowed them to weather through some early defeats and setbacks in order to embrace the larger goal of victory. The Southern mentality played into this. They figured that the conflict would be a quick one, a relatively simple affair where their superiority in training and war fighting tradition would scare off the Northerners. It seemed that the Southern mentality which embraced a quick win and a Northern one that embraced a prolonged conflict were complements to one another, and a set that played out in a Northern victory and a Southern defeat.
The greatest advantage the Union had in fighting the Civil War was definitely population. The Union had about 22 million people while the Confederacy had about 9 million, with over 3 million of those people being slaves. When Ulysses Grant took over as commander of all Union forces, he realized the potential of this population difference and began fighting a war of attrition. He engaged the enemy whenever he could. He knew that he could replace his losses while the Confederacy could not. This was a brutal and bloody strategy, but it worked.
Perhaps the greatest weakness the Confederacy had was a lack of any major European ally. As the war began, southern leaders felt that Great Britain would join them in their fight against the Union. The Confederate leaders believed this because they thought that Britain would need their cotton for the British textile industry. If Britain had joined with the Confederacy, then the Confederacy would have had more fighting men, a navy, an industrial base in Britain, and more money to fight the war—perhaps completely changing the outcome of the war.
Agreed with the previous post. The North also had the vast majority of factories, and the vast majority of rail lines for transportation. When you coupled that with their overwhelming advantage in numbers, they were pretty much unstoppable. Amazingly they still almost managed to blow it. They had very poor military leadership in the field for the first two years of the war, but then Grant and Sherman emerged as champions.
Southerners had no Navy, which was a huge factor in their loss, because they could not consistently break the blockade that prevented them from obtaining foreign aid or selling cotton. They were fighting at more than a 4 to 1 disadvantage, and to win the war had to successfully defend the entire Confederacy, without the forces, unity or materials to ever do so. Despite all of this, they managed to hang on for four years, and inflict obscene casualties on the Union armies.
In my opinion, the Northern strength that led to victory was its advantage in men and material. The North had many more people, especially when you consider that the South had a large slave population that could not really help them militarily. The North also had more industry and better railroads.
I think that the main Southern weakness was its desire to fight an offensive war. I think that they could have prolonged the war a great deal if they had fought on the defensive from the outset. Particularly, if they had not allowed the taking of New Orleans, they would have held on to the Mississippi for much longer. A prolonged war would have been more likely to result in a peace treaty and/or in recognition of the Confederacy by European countries.