The Oglala Sioux have joined the legal battle against the U.S. Army Corps of Engineers, filing a lawsuit this week calling on the agency to halt construction of the Dakota Access Pipeline (DAPL) until an environmental review can be completed.Tribal leaders cited fear of water contamination in their lawsuit, which was filed Monday in the U.S District Court in Washington, the same day a federal judge rejected an emergency request from the Standing Rock Sioux and the Cheyenne River Sioux tribes to delay construction.The action comes ahead of the corps pledging to evict anyone still camping at the main resistance site in Cannon Ball, North Dakota, on February 22.The corps granted DAPL\u0026#039;s parent company its final easement this month and canceled the Environmental Impact Statement (EIS) amid widespread protests.\u0022After the Corps granted the easement for the DAPL to cross under Lake Oahe and stopped the environmental impact statement process, we really had no choice but to go to court to protect our rights under the 1851 and 1868 Fort Laramie Treaties to safe drinking water,\u0022 tribal President Troy \u0022Scott\u0022 Weston said in a statement Monday.The Oglala Sioux are situated in South Dakota, one of the four states the 1,172-mile pipeline crosses. The lawsuit warns that a spill in the Missouri River would violate Indigenous treaty rights, contaminate clean water for more than 200,000 state residents, and bring about other environmental devastation.\u0022The waters of the Missouri River are sacred to the tribe,\u0022 the Oglala Sioux\u0026#039;s lawsuit states. \u0022These waters give life to all of the creatures and plants on the tribe\u0026#039;s lands. The tribe has treaty- and statute-protected property rights to the waters of the Missouri River.\u0022President Donald Trump, who took office pledging support for Big Oil, signed an executive order last month instructing the corps to expedite the DAPL approval process—reversing an effort by former President Barack Obama to halt construction until the EIS could be completed.Kelcy Warren, CEO of DAPL\u0026#039;s parent company, Energy Transfer Partners, has previously referred to the water protectors and other pipeline opponents as terrorists.Resistance to the pipeline has been ongoing since last summer, with more than 200 tribes from around the country supporting the front-line efforts in North Dakota led largely by the Standing Rock Sioux. That tribe was also in D.C. court this week, filing a motion to overturn the Lake Oahe easement.