How & why did the North win the Civil War?
When the Civil War began, the North enjoyed enormous material advantages. The bulk of the nation's industry was in states that remained in the Union, as was the vast majority of the nation's population. Additionally, the North had significant infrastructural advantages over the South, particularly in railroad construction. Despite these advantages, it was never inevitable that the North would win, especially in light of the war aims of each side. Where the Union armies had to conquer and hold Confederate territory, the South only had to make the war too costly to pursue in the minds of Northern people.
Until 1863, and even, some might argue, one year later, there was much evidence that the Southern armies might achieve this goal. They won victory after costly victory, especially in Virginia, taking advantage of bungling Union generals to inflict terrible losses on the Federal Army. The North was able to win the war by seizing key strategic points in the West, thus closing the Mississippi river; maintaining a firm blockade on Southern ports, depriving them of valuable commerce and essential war material; keeping European powers from joining the war; and finally by grinding down the Confederate Army outside Virginia. None of these objectives were foreordained, and for much of the conflict, the outcome of the war was finely balanced.