Star Trek: Nautilus
The Tomed Incident is highly classified. It is not widely known even what the “Tomed” in the incident refers to. Most believe it refers to the Tomed System in Federation space near the Romulan Neutral Zone. Some believe it refers to a Romulan Vessel. Still others think Tomed is a person. All that is widely known is that the incident led directly to the Romulans seeking peace and isolation from interstellar politics.
Some point to the clause involving cloaking technology in the Treaty of Algeron as evidence that the Tomed Incident involved the Federation 8th Tactical Fleet and specifically the Excelsior-Class starships stationed at Starbase 23 equipped with cloaking devices, but it is also likely the the restriction on cloaking technology was to offer the Romulans an added layer of privacy during their isolation.