Shein and Forever 21 announced a deal that will combine the fast-fashion competitors’ customer reach. As part of the joint venture, Shein will acquire a third of Forever 21’s operator, SPARC Group, and SPARC Group will take a minority stake in Shein.

The rapid rise of these two China-based companies has shaken up the way Americans shop online, as they have successfully challenged and ultimately overtaken behemoths like Amazon, eBay and Walmart. But with rising economic tensions between the U.S. and China, troubling environmental concerns and pending litigation between the two companies, the sustainability of their success isn’t a given.