Nexus in Law: How Causal Links Are Established in Benefits Claims

The VA benefits process is one of the most misunderstood systems veterans ever navigate.

And here’s the truth…

A well-documented claim with great medical evidence can still be denied. Not because the condition doesn’t exist. Not because the service connection isn’t provable. But because one thing is missing.

That piece is nexus.

What you’ll dig into:

1. What Is Nexus in Law?
2. Why the VA Benefits Process Depends on It
3. The Three Types of Service Connection
4. What a Nexus Letter Actually Is
5. How Causal Links Are Built and Evaluated
6. Common Reasons Nexus Claims Fail

What Is Nexus in Law?

Originating from Latin, Nexus translates to “connection” or “link.” Legally speaking, it means the causal connection between two events. When seeking VA benefits, it refers to the proof of direct connection between a veteran’s medical condition and their military service.

Without that link, the claim has no legal foundation.

Seriously. That’s it. If there is no written medical evidence linking a condition to service, VA will not pay compensation. Whether it occurred during service or after service, or was aggravated by service.

Why the VA Benefits Process Depends on It

Nearly 36% of VA disability claims were denied in fiscal year 2024 — out of over 2.4 million processed claims. Which means hundreds of thousands of veterans are left without benefits they deserve.

And one of the most consistent reasons for denial?

A missing or weak nexus.

Lack of a clear medical nexus is the number one reason VA claims are denied for not being service-connected. Veterans can have a diagnosis. They can have in-service records supporting the condition. But if there’s no proof that the two are connected, the VA will have no legal reason to approve the claim.

Claim Climbers delivers evidence-based Nexus Letters that are supported by evidence AND worded to satisfy the VA’s service connection requirements using the medical standard of proof the VA uses: “at least as likely as not.”

The Three Types of Service Connection

It all starts with service connection. There are three ways to establish:

Direct Service Connection — The injury or illness was caused by something that happened while on active duty. For example, hurting a back lifting heavy equipment, or injuring a knee in a training accident.

Secondary Service Connection — The condition was not caused by service but was caused as a result of another service-connected condition. Example: Depression as the result of service-connected chronic pain.

Aggravation — The veteran had a condition prior to service that was made measurably worse during active duty. Difficult to prove but possible under VA law.

Each still must flow through nexus. The nexus connection is different. Requirement for written cause still applies.

What a Nexus Letter Actually Is

A nexus letter is a written opinion from a qualified healthcare professional. It states, in medical terms, how the veteran’s present condition relates to their military service.

Here is what a strong nexus letter needs to include:

• The veteran’s diagnosis
• A review of their service records and medical history
• A clear medical rationale connecting the two
• Standard of proof of “at least as likely as not” (51% or better chance of relationship)
• The provider’s credentials and signature

Simple enough, right? Written down. The problem is most veterans don’t know how to obtain one — or why the letter’s quality is so important.

An incomplete, one-paragraph opinion is worth pennies on the dollar. A lengthy, well thought out medical opinion that ties the service event(s) and clinical literature together? Priceless.

How Causal Links Are Built and Evaluated

When determining whether nexus exists, the VA applies a specific legal standard. The Caluza Triangle provides three elements:

1. A current diagnosis
2. An in-service event, injury, or disease
3. A medical nexus linking the two

All three elements must exist in order for a claim to stick. Remove or weaken one element and the claim will fall apart.

The strength of the connection is measured by probative value. Not every medical opinion is created equal. VA considers the overall merit of the opinion taking into consideration how reasoned the opinion is, the qualifications of the provider, and how much the provider addresses the veteran’s individual case.

That last part is important. Form letter type submissions that do not address the veteran’s service records will be tossed out. Letters that discuss events, include medical literature, and address a records review will be given merit.

Common Reasons Nexus Claims Fail

Considering the significance of this evidence, it bears exploring where things go wrong.

The most common reasons nexus claims fail in the VA benefits process include:

• No medical opinion at all — Veterans think their diagnosis will speak for itself, but the VA needs a clear connection of the dots.
• Weak or vague language — Letters indicating “possibly related” or “may be connected” do not rise to the level of “at least as likely as not.”
• Wrong provider — Not all doctors understand VA criteria. Evidence submitted by a provider who is unfamiliar with what the VA is looking for is likely to be considered low-value evidence.
• No engagement with service records — The letter must demonstrate that the provider reviewed the veteran’s military records, not merely their diagnosis at present.
• Missing credentials — The VA evaluates the credibility of the source. Credentials matter.

Between 60–70% of denied VA claims that are appealed successfully include proper supporting evidence. One of the keys to getting a VA denial overturned is a well-written nexus letter.

Wrapping Things Up

Nexus isn’t some jargon VA thing. It’s the backbone of every service-connected claim.

Without it, even the strongest meritorious case will fail. With this piece completed correctly — written, documented, and argued — the entire claim is rock solid.

To quickly recap:

• Nexus establishes the legal and medical connection between a current condition and military service
• The VA requires all three elements: diagnosis, in-service event, and nexus
• The strength of the nexus letter will determine how heavily the VA values it
• Lack of nexus is the number one reason service-connection claims are denied

A properly constructed nexus letter that reaches the “at least as likely as not” standard allows the claim its best opportunity.

Every veteran who served should be given an equal opportunity to receive the benefits they earned. Fighting for them starts with getting nexus correct.