"They were setting up a military base?"
"It was the only logical conclusion."
"Was there any evidence?"
"How much evidence would you need if the Klingons decided to set up a "colony" on Pluto?"
Weytahn was the name given by the Andorians to a formerly uninhabitable class D planetoid, approximately as large as Earth's moon. It was the home to a rather pernicious mutagenic pathogen that was capable of harming Humans, but did not affect Vulcans. Weytahn was located on the frontier between the Vulcan system and the Andorian system. Both sides claimed it belonged to them, having nearly gone to war over the planetoid twice.
Weytahn was terraformed by the Andorian Empire in the 2050s for the purpose of establishing a colony there. Once an atmosphere was developed, they established a settlement. Due in part to its relatively close proximity to Vulcan territory, Weytahn was believed by the Vulcan High Command to be a covert military installation.
Vulcans and Andorians eventually reached a Territorial Compromise, which took eight years to draw up and was later viewed by Andorians as the source of a century of aggression.
In 2097, the refusal of the settlers to let the Vulcans inspect their colony provoked the Vulcans into annexing the planet, and it was renamed Paan Mokar. Weytahn's settlers were forcibly removed and placed in refugee camps. The new situation was codified in the Treaty of 2097. A surveillance satellite was put into orbit to monitor the agreement.
Weytahn remained deserted until 2152, when Shran, an Andorian regimental commander, landed a force on the planetoid and occupied the settlement in an attempt to forcibly rescind the Compromise. The Vulcans attempted to enforce the Treaty of 2097 and managed to regain control of half of the colony. They, however, suffered several casualties, including two dozen wounded and three taken hostage. Shran agreed to discuss terms for their safe return, but only if Jonathan Archer would act as mediator. Archer was able to get the two sides together and establish a common goal for them. (ENT: "Cease Fire")
The Vulcans eventually agreed to cede ownership of the world to the Andorians. Two years later, when High Command Administrator V'Las planned an invasion of Andoria, the Andorians were tricked into believing the Vulcans would attempt to retake Weytahn, thereby committing much of their fleet to that world and leaving Andoria vulnerable. (ENT: "Kir'Shara")