The Treaty of Horizon was ratified in 2399 and solidified cooperative relations between the Federation and the Romulan Empire. It is an integral document for both governments.
The 2399 Treaty of Horizon is included here in full text.
2399 Treaty of Horizon
From: Federation Liaison Department (Delta Quadrant)
To: Commodore Gruber (Delta Quadrant),Captain O’Reilley (CO, Starbase Horizon), Major Henry (XO, Starbase Horizon), Romulan Embassy (Starbase Horizon), Klingon Embassy (Starbase Horizon), Ferengie Embassy (Starbase Horizon)
Treaty of Horizon by and between the Romulan Star Empire and the United Federation of Planets
The terms and conditions having been negotiated and agreed to be identically understood. They allows for mutual aid and defense against aggressors, but forbids interference in the internal affairs of either government.
No vessels under the auspices of the Federation, Klingon Empire, and Romulan Star Empire would fire upon or exchange fire upon the other. Any commander, or individual, or group, that would do so would be court martialed, or tried, for treason and acts of piracy, and face the full punishment under law.
Intelligence and technology beneficial to the United Defense Force would be shared without restrictions, after the delegating authority has factored out any possible risks to their government's national security.
Weapons of mass destruction are to be used as deterrence, or as defensive measures against advanced hostile races when ship and crew are a risk of being destroyed. Such weapons shall not be used as in an offensive manner unless a state of war exists.
Any non-Romulan vessel sharing in the defense of Star Empire space bordering an alien hostile region shall be required to accept one (1) Romulan observer sent as a liaison between the non-Romulan vessel and Star Empire. The acceptance of any individual observer is subject to the approval of the captain of the vessel. However, the captain may not reject any observer-applicant on unreasonable grounds. Also any Star Empire ships shall be required to accept Klingon and Federation observers aboard when they were sharing in the defense of Klingon and Federation space, with the captains of the Romulan vessels given the same rights of approval.
Violation of treaty terms will be dealt with on a case by case basis under previously instated laws and regulations.
I, Lill'vahe t'Eilairivieu, having read the terms of this treaty verbatim, and having confirmed the translation of dates and frequencies from Terran Standard and having been fully identified by voiceprint scan, do certify that I am Romulan Ambassador to the Federation at Starbase Horizon in Delta Quadrant and am empowered to conclude this Treaty on behalf of the Romulan Star Empire.
I, Relian Kru, having read the terms of this treaty verbatim, and having confirmed the translation of dates and frequencies from Romulan Standard and having been fully identified by voiceprint scan, do certify that I am Federation Liaison of the Delta Quadrant and am empowered to conclude this Treaty on behalf of the United Federation of Planets.
Done 1. October 2399