The company building the long-contested Keystone XL oil pipeline notified the Cheyenne River Sioux Tribe in a letter this week that it will start stockpiling equipment along the pipeline's route this month in preparation for construction.

Tribal Chairman Harold Frazier sent back a sharp, one-line response: "We will be waiting."

The Cheyenne River tribe has opposed the Keystone pipeline since it was first proposed in 2008, and it has seen how pipeline protests can play out.

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Just to the north of the tribe's land in central South Dakota, protests against the Dakota Access pipeline drew international attention as thousands of demonstrators established semi-permanent camps starting in the summer of 2016 near where the pipeline would cross under the Missouri river just upstream from the Standing Rock Indian Reservation.

The Cheyenne River tribe helped fight that pipeline, and it is in a similar geographic situation now—its reservation is just downstream from where the Keystone XL pipeline would cross the Cheyenne River.

Like Standing Rock, the tribe fears a tar sands oil spill from the pipeline could contaminate its waters.

While TransCanada still faces some challenges to its permits for Keystone XL, it wrote to Frazier that it expects to start construction in 2019.

"Chairman Frazier wanted to send a clear signal that they are not welcome," Remi Bald Eagle, intergovernmental affairs coordinator for the Cheyenne River tribe, said.

TransCanada did not respond to a request for comment.

Frustration Over Pipeline in Indian Country

Similar sentiments are being expressed across stretches of Indian Country this summer as TransCanada and Enbridge prepare to build two separate pipelines that would ship tar sands crude oil from Canada into the United States. Tribes and environmental organizations oppose both projects, citing concerns that oil spills would contaminate surrounding waters.

When the Minnesota Public Utility Commission voted in June to approve a new Enbridge Line 3—a proposed 1,000-mile pipeline running from Hardisty, Alberta, to Superior, Wisconsin—Tania Aubid, a member of the Mille Lacs Band of Ojibwe, shouted at the commissioners: "You have just declared war on the Ojibwe!"

The growing frustration raises the prospect of large-scale demonstrations again, perhaps similar to those that drew thousands to protest construction of the Dakota Access pipeline in North Dakota starting in 2016. By late fall of that year, pipeline opponents had clashed with increasingly militarized law enforcement. The protests ended in the forced removal of the camps after the pipeline was given final federal approval in early 2017.

Floyd Azure, chairman of the Fort Peck Assiniboine & Sioux Tribes on the Fort Peck Reservation in Montana, also received a letter from TransCanada on July 11 outlining the company's plans to begin bringing in supplies and clearing areas along the route. He fears TransCanada's actions will spark protests near his tribe's reservation, as well.

"I don't wish it to be like that, but it will probably end up like that," Azure said. "I don't feel we should have to go through what Standing Rock did. We jeopardize not only the people of the Fort Peck reservation, but we also jeopardize the workers on the pipeline."

Tribe 'Fully Prepared to Support' Resistance

The Cheyenne River Sioux Tribe, along with the Yankton Sioux Tribe and others, challenged South Dakota's approval of the Keystone XL pipeline in court, but the lawsuit was dismissed by the state Supreme Court in June.

What comes next remains unclear.

The tribal council passed a resolution in 2014 to prohibit TransCanada from moving equipment through the reservation to build its pipeline, and it plans to discuss how it would fund enforcement, Bald Eagle said. "The Tribal Council reserves the right to exclude and escort off anyone on the reservation," he said.

Bald Eagle said he didn't anticipate on-the-ground opposition to the pipeline until construction begins in earnest, but added that the tribe would be ready to support such opposition.

"I can't speak about what is going to happen—obviously different groups have different ideas about what they are going to do—but the Cheyenne River Sioux Tribe is fully prepared to support any of its tribal members in any of their efforts to resist the pipeline," he said.