Direct service connection rests on three pillars. The VA calls them the elements; I call them the Big Three. You need all three, and the whole claim is only as strong as the weakest one.
1. A current diagnosis
You must have the condition now, documented by a medical professional. “I used to have it” does not pay.
2. An in-service event, injury, or exposure
Something in service that could have caused the condition — an injury, a stressor, a toxic exposure. Service records, buddy statements, and unit histories build this.
3. A medical nexus
A competent opinion linking the diagnosis to the in-service event, at the “at least as likely as not” standard. This is the pillar the VA attacks most, and the one a strong nexus letter is built to defend.