"States Party to" or "State Parties to" or "States parties to"?

Solution 1:

The first two are both grammatical, and mean roughly the same thing, but are parsed slightly differently.

100 states party to the treaty

party to the treaty is being used as an adjectival phrase that describes the 100 states.

100 state parties to the treaty

parties is a noun, qualified by state, and to the treaty is a prepositional phrase that describes these 100 state parties.

Google Ngrams shows that states party to the treaty is slightly more common, although they've swapped positions a number of times over the past few decades. It sounds better to my ears, but that's just personal opinion.

100 states parties to the treaty

can't be parsed in either way. When a noun is used as a qualifier, it's normally singular. And parties is not an adjective, only party would be.

Solution 2:

States parties is a term of art in public international law. It refers to the states that have signed a particular treaty. For example: states parties to the Treaty of Versailles.

It may not make sense from a purely grammatical perspective, but it is regularly used in international law. The first and second options are not commonly used in international legal material and would probably considered wrong if you are writing for a diplomatic or legal audience.

As an example: UNESCO, States Parties Ratification Status

States Parties are countries which have adhered to the World Heritage Convention.They thereby agree to identify and nominate properties on their national territory to be considered for inscription on the World Heritage List

For reference, explained by Karel:

In international law it is a term of art as stated above. You find examples of this in international treaties such as Article 4(1) of the International Covenant on Civil and Political Rights (ICCPR) which reads:

"In time of public emergency which threatens the life of the nation and the existence of which is officially proclaimed, the States Parties to the present Covenant may take measures derogating from their obligations under the present Covenant to the extent strictly required by the exigencies of the situation, provided that such measures are not inconsistent with their other obligations under international law and do not involve discrimination solely on the ground of race, colour, sex, language, religion or social origin."