The Polaric Test Ban Treaty was signed in 2268, which banned research into polaric ion energy.
The treaty was precipitated by the near-destruction of the Romulan research colony on Chaltok IV in a polaric explosion. (VOY: "Time and Again")
Although the signatories of the Polaric Test Ban Treaty have not been explicitly mentioned on screen, the context of the respective line in "Time and Again" as well as the immediately preceding diplomatic development on Nimbus III might suggest that the Romulan Star Empire and the Federation were among them. The Star Trek Encyclopedia, 4th ed., vol. 2, p. 173 named these two major powers as signatories to the treaty.