Texas v. New Mexico (Decision March 5, 2018)
The United States can get a piece of this water dispute between Texas, New Mexico and Colorado.
Generally, only the parties that signed a contract can sue on the contract. If my cousin gets evicted and I was crashing with him, I can't sue the landlord to argue the landlord violated the lease. I wasn't a party to the contract.
However, certain contract beneficiaries or other parties properly related to the contract can try to get involved.
In this case, the Supreme Court was asked to decide whether the United States had the right to intervene in the lawsuit between Texas, New Mexico and Colorado.
The Rio Grande Compact
In 1938, Texas, New Mexico and Colorado signed a contract (the "Rio Grande Compact") to apportion the waters of the Rio Grande fairly. Colorado was to deliver a certain amount of water downstream to the New Mexico state line, and New Mexico would deliver water for Texas to a United States reservoir, the Elephant Butte Reservoir, which was in New Mexico.
The Elephant Butte stop was guaranteed to work for delivery into Texas because the United States (federal government), which operates Elephant Butte, had already agreed to help deliver water from there to Texas.
United States' additional relationship
The federal government has an additional relationship to the water contract. In 1906, the United States had agreed in a treaty with Mexico that it would guarantee a certain amount of water all the way down to Mexico. If New Mexico is stopping the water from getting to Texas, as Texas argues, the United States would have a harder time satisfying its treaty obligations.
The Supreme Court ruled that the United States was closely-enough tied to the Compact and affected by the Compact that it can intervene in the litigation.
The court did note, however, that the case is a very special case. It is one in which the Supreme Court has original jurisdiction, which gives the Supreme Court a special power to invite the federal government to assert its "distinctively federal interests."