A non-aggression pact is a treaty between two or more governments agreeing to not engage in open warfare against each other.
In late 2373, after the Cardassian Union joined the Dominion the Dominion made significant political inroad into the Alpha Quadrant by signing several non-aggression pacts. The Tholians, Miradorns, Bajorans and the Romulan Star Empire were among the governments to sign these treaties. This basically left only the Federation and Klingon Empire to openly oppose the Dominion's actions in the quadrant. (DS9: "In the Cards", "Call to Arms"; The Dominion War Sourcebook: The Fires of Armageddon)
The Garanog who resided in the Kalandra sector signed a non-aggression pact with the Dominion and in exchanged helped with the Dominion's supply lines. (The Dominion War Sourcebook: The Fires of Armageddon)
The Romulan Star Empire would break their non-aggression pact with the Dominion in mid-2374. After discovering evidence that the Dominion was planning an invasion of the Star Empire and the assassination of Senator Vreenak, which they blamed the Dominion for. The evidence was in fact fake and the death of Vreenak was due to a bomb planted by the Cardassian Garak in a plot to draw the Romulans into the war on the Federation's side. (DS9: "In the Pale Moonlight"; The Dominion War Sourcebook: The Fires of Armageddon)