Previously, sanctions and money laundering investigations, from a U.S. perspective, were usually limited to individuals or families of transactions that possesses a U.S. component to them. 

For example, a transaction from, to, or through foreign banks might only fall under U.S. enforcement jurisdiction if it passed through a U.S. financial institution, used the U.S. dollar, or had a U.S. person or business entity acting as an originator, beneficiary or facilitator.

From the ruling:

"Correspondent banking arrangements are fairly ubiquitous: “[d]ue to U.S. prominence in international trade and the high demand for U.S. dollars due to their overall stability, most foreign banks that wish to provide international services to their customers have accounts in the United States capable of transacting business in U.S. dollars.” Id. In short, then, correspondent bank accounts allow foreign banks to offer their clients services—including obtaining and transacting in U.S. currency—without opening a U.S. branch.

In this case, however, the United States chose not to go through the MLAA. Stymied by what it perceives as sluggish and typically fruitless cooperation from Chinese authorities in the past, the government unilaterally issued subpoenas to all three Banks in December 2017. The two banks with branches in the United States (“Bank One” and “Bank Two,” respectively)—received grand jury subpoenas. See Federal Rule of Criminal Procedure 1 7(e)(1). The third (“Bank Three”)—has no branch in the United States and received a subpoena from the Attorney General pursuant to the Patriot Act, which gives the Attorney General and the Treasury Secretary special investigatory tools when it comes to U.S. correspondent bank accounts."