Service connection is the VA’s recognition that your current condition was caused or aggravated by military service. A denial for lack of service connection means the VA found insufficient evidence linking your diagnosis to your service. Challenging this requires new medical evidence, typically a “nexus letter” from a medical professional who reviews your records and provides an opinion establishing the connection. An attorney helps find the right medical expert, properly frame the nexus question, and submit the supplemental claim with a strong foundation.