India has purchased a new missile system from Russia, putting it at risk of U.S. sanctions.

What are the details?

On Friday, India finalized a deal to buy a S-400 missile defense system from the Russian government for $5 billion. The two nations have been in talks regarding this deal since 2015.

The S-400 is one of the most advanced surface-to-air missile systems in the world. It is able to detect and destroy 80 targets at once. The Times of India quoted Indian Air Force Chief BS Dhanoa as saying that this purchase would serve as a "booster shot" for the Indian Air Force.

The U.S. has already imposed sanctions on China after that nation bought the same type of missile system from Russia in December and January. The sanctions targeted China's Equipment Development Department and its director, Li Shanfu, for “engaging in significant transactions” with people on the department’s List of Specified Persons.

Li and the EDD were both banned from U.S. financial systems, denied export licenses, and added to the List of Specified Persons, a list of people and entities deemed by the U.S. government to be associated with “or operating for or on behalf of, the defense or intelligence sectors of the Government of the Russian Federation.”

Last August, President Donald Trump signed the Countering America's Adversaries Through Sanctions Act, which slapped sanctions on Iran, North Korea, and Russia, and allowed sanctions to be imposed on nations who had business dealings with anyone on the List of Specified Persons.

Anything else?

Trump could provide India with an exception to this rule, but that could open the door for other nations to demand that they also deserve exceptions.

While China is largely seen as a rival, India is an important U.S. ally. After the deal was finalized, a spokesperson for the U.S. embassy in India told CNN that the purpose of CAATSA was not to "impose damage to the military capabilities of our allies or partners."

That spokesperson declined to say definitively whether or not the U.S. would consider punishing India for the purchase.