From the writings of the historian Stefan of Ambrosia, 26 AA:
The Nine Powers Conference assembled on 22 November 2 AA, on the fifth moon of the superjovian Pacem in the Crossroads system. Present were delegations from all nine polities capable of starship construction. After the horrific destruction and slaughter of the First Great Inter-Stellar War, nations sought to impose limits upon the military of their fellows, in the vain hope that future wars could be avoided, or at least kept manageable. It reality, it managed only to ignite a new arms race of technology instead of mass, and weakened the signatories to the point that the Second Great Interstellar War nearly annihilated all of them. The terms of the Nine-Power Treaty were agreed to on 6 February 3AA, and announced simultaneously (after a lengthy ratification process) from the signatories' respective capitals on 17 August 4 AA.
After the Armistice, the Empire of the Lion had the galaxy's largest and most powerful fleet, followed closely by the Wichita Confederation, and more distantly by the Empire of the Rising Sun and the Theocracy of Panj. All four embarked upon large programs of new battleships. In 1 AA, the Wichita Confederation had declared an aim to produce a fleet "second to none", and had already laid down five battleships and four battlecruisers. The Empire of the Rising Sun was at the start of an 8-8 program (eight battleships and eight battlecruisers). In early 2 AA the Empire of the Lion finalized the design of and ordered four very large battlecruisers with plans for four matching battleships to follow. The Theocracy of Panj, immense in number of hulls but lacking in large ships, purchased old Lion designs and prepared to build twenty-two battleships & battlecruisers. This burst of construction kindled fears of a new naval arms race, similar to the Lion-Iron League race preceding The War.
At the time, the Wichita Confederation's economic power was considerably greater than that of its potential rivals. Its gross domestic product was approximately three times larger than the Lion Empire's (excluding its minor allies & possessions), six times larger than the Empire of the Rising Sun, and seven times larger than that of the Theocracy of Panj. While the Wichita Confederation had the wherewithal to outbuild the other powers, its corporate oligarchy meant that domestic political support for such an ambitious program was lacking. The Lion Empire and Theocracy of Panj were linked by the Leonine-Panjubi Alliance which included mutual defense. The prospect of a naval limitation treaty offered the Confederation government a chance to ?get on with business? while offering the Rising Sun, Leonine, and Panjubi governments a more favorable balance of power compared to the Wichita Confederation than they could have achieved on the building ways. The discovery shortly before The War's end of a 'back door' warp line linking outlying Lion & Confederation systems with outlying systems of the Empire of the Rising Sun likely played a large part in their agreement to attend the conference. Their days of ?splendid isolationism? behind the bulk of the Commissariat of Tsarkov were over.
In general, the terms of the treaty caused a shift away from large ships with large guns to a greater number of smaller ships with smaller guns, and parasites. The treaty limited the total tonnage of 'capital ships' (a new designation, for the first time given a precise definition), the maximum tonnage of individual ships, the total tonnage of parasite motherships (given the new term 'carriers'), the maximum size & number of guns of a carrier, the maximum size of gun to be placed on any ship, the building and/or provision of any armed ship to any non-signatory nation, the disposal of armed ships, and the building and/or upgrading of static defenses outside of a nation's most heavily populated systems.
The overall form of the treaty was a capital ship & parasite displacement ratio of 10:10:8:6:6:5:5:4:4, with the Wichita Confederation & the Empire of the Lion predominant, the Empire of the Rising Sun secondary, and the Theocracy of Panj reduced to parity with the Commissariat of Tsarkov. The ever fractious neighbours of Le Troisieme Respublique and the Stella Regia de Medeo were evenly matched, and the defeated powers of the Iron League and the Caliphate Ottomo restricted to the smallest fleets of all. The Commissariat of Tsarkov bitterly resented being restricted to a fleet smaller than that of its neighbour the Empire of the Rising Sun, despite having four times the population and nearly twenty times the total number of populated systems. Indeed, the massively spread out, thinly-populated Commissariat presented several unique challenges, and at the end was persuaded to sign only upon receipt of several key technological boons and sales of numerous smaller ships ?for internal security purposes?.
The treaty optimistically sought to restrain the technological arms race that, ironically, it directly inspired by restricting new capital ship construction to that required to replace existing ships (save those lost to accident or conflict) not less than twenty years after construction. It was felt that this would result in a gradual increase of capability rather than the pre-War tendency to lay down a half-dozen new capital ships with each technological advance. In practice, existing ships were extensively refit with newer technology whenever governments found their capabilities lacking. The most notable example being the Confederation's Kansas and Nebraska, which had their engines replaced in 6 AA, their electronics upgraded in 7 AA, their guns replaced in 10 AA, their electronics upgraded again in 11 AA when it was found that their new guns exceeded expectations by a wide margin, their engines replaced again in 12 AA, their shield generators replaced in 14 AA, and were stripped to the keel to have their armour and engines replaced in 17 AA. Finally, Kansas and Nebraska had their guns, electronics, and shield generators replaced in 22 AA, just in time to be destroyed at the battle of Diamond Valley in December 22 AA.
Alas, one important area was completely overlooked. Missile combat was in its infancy in The War, with missiles being large, unreliable, and very inaccurate. It was felt at the time that a properly handled ship was in no danger from missiles, and as a result no efforts were made to limit the size or number of missile launchers on a ship. That fact was to have dire implications in the Second Great Inter-Stellar War.