The VA disability system is unusual in American law: it is non-adversarial by statute, it must help the claimant build the case, and ties go to the veteran. This article explains what service connection actually requires, why the nexus opinion is the element that decides nearly every contested claim, and how the presumptions work for those who cannot prove causation directly. It covers ratings, effective dates, and the three appeal lanes that replaced the old system. It follows two fictional veterans through the same evidence with very different outcomes. The practical thesis is that most denied claims fail on a missing medical opinion or a missing effective-date argument, not on the merits of the disability.