Answer: Manpower/soldiers
Step-by-step explanation:
The Union had a population of 25 million+ when the Civil War broke out compared to the Confederacy's 5 million+ and so were able to call forth a larger army than the Confederacy.
Union General Ulysses S. Grant used this to devastating effect when he faced off against General Lee of the Confederacy. Time and Time again Lee inflicted heavy damage on Grant but Grant would order his troops forward knowing full well that Lee could not replenish his army like Grant could.
This ended with Lee surrendering to Grant.