Why should people be cautious in making claims that northern victory was a foregone conclusion

It took three years before the north finally put Grant's plans in to full action. Between 1861 and 1864 the South had a decent opportunity to force the Union to surrender. The Union did, in fact, lose many battles during the first two years so the Union losing the war was not unheard of. While the North believed the had God on their side because they were ending slavery, for men who were simply enlisted and made to fight, there was little motivation for them to win the war. Once the slaves were free, they would have more people to compete with them for jobs and money was scarce for some of these people. Those fighting for the Confederacy were fighting for their hearts and way of life, they believed they were fighting for their very freedom, which meant those in the south were likely to fight much harder than those in the north.