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I am sorry to hear about this situation. It sounds like the organization was told and/or has reason to believe that the US Army owns that land, but you say you do.
What someone in your situation wants to do is to threaten (or file) suit in Court to get this matter resolved. To sue in a state court, one needs to have a "cause of action." There are numerous causes of action, such as "breach of contract," "negligence," "fraud," "unjust enrichment," etc., as well as causes of action rooted in statutory law. Every state has their own although they are very similar to each other in every state because they all stem from the same common law. A pleading in Court needs at least one cause of action, although it is not unusual to have more than one.
Herein, several causes of action come to mind:
CIVIL TRESPASS against the organization. Self explanatory.
NUISANCE against the organization.
One is subject to liability for private nuisance if, but only if, his conduct is a legal cause of an invasion of another's interest in the private use and enjoyment of land, and the invasion is either:
(a) intentional and unreasonable, or
(b) unintentional and otherwise actionable under the rules controlling liability for negligent or reckless conduct, or for abnormally dangerous conditions or activities.
Diess v. Pennsylvania Dept. of Transp., 935 A. 2d 895 - Pa: Commonwealth Court 2007, citing 2nd Restatement.
QUIET TITLE - this is a cause of action to ask the Court to determine who owns the property and controls it, and by extension, if the easement should be allowed or voided.
While the suit is pending, one can ask for temporary injunction from use the easement from the Court, as well.
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