Army Corps Will Not Grant Easement for Dakota Access Pipeline Crossing At least 2,000 veterans arrived at the protests this afternoon.

 -- The Department of the Army will not approve an easement that would have allowed the proposed Dakota Access Pipeline to cross under Lake Oahe in North Dakota, the Army's Assistant Secretary for Civil Works announced today in a statement.

The Army claimed in a statement that Assistant Secretary for Civil Works Jo-Ellen Darcy based her decision on a need to explore "alternate routes for the Dakota Access Pipeline crossing."

Native American groups and environmental activists have been protesting since summer to block construction of the 1,172-mile pipeline that would have cut across four states and transport crude oil from North Dakota's oil fields to refinery markets in Illinois. The activists, who call themselves "water protectors," say that the pipeline traverses culturally sacred sites and poses a risk to the Standing Rock Sioux Tribe's water supply.

"Although we have had continuing discussion and exchanges of new information with the Standing Rock Sioux and Dakota Access, it's clear that there's more work to do," Darcy said, regarding the decision. "The best way to complete that work responsibly and expeditiously is to explore alternate routes for the pipeline crossing."

Secretary of the Interior Sally Jewell released a statement in support of the decision, saying that the "thoughtful approach established by the Army today ensures that there will be an in-depth evaluation of alternative routes for the pipeline and a closer look at potential impacts."

The Standing Rock Sioux tribe also praised the decision, and thanked both the Obama administration and the many people who supported the effort to stop the pipeline from being built across Lake Oahe.

"We wholeheartedly support the decision of the administration and commend with the utmost gratitude the courage it took on the part of President Obama, the Army Corps, the Department of Justice and the Department of the Interior to take steps to correct the course of history and to do the right thing," Standing Rock Sioux Tribal Chairman Dave Archambault II said in a statement. "The Standing Rock Sioux Tribe and all of Indian Country will be forever grateful to the Obama Administration for this historic decision."

Jan Hasselman, the tribe's attorney added, "today is a historic day both for the Standing Rock Sioux Tribe and for indigenous people everywhere. This pipeline never should have been routed near these sacred lands in the first place, and it absolutely never should have received permits without a thorough and meaningful discussion of the risks and benefits to affected Indian Tribes."

Senator Bernie Sanders applauded the Army Corps decision today, saying in a statement "in the year 2016, we should not continue to trample on Native American sovereignty. We should not endanger the water supply of millions of people."

North Dakota lawmaker Kevin Cramer, however, called the Army Corps announcement an "unfortunate decision."

"I hoped even a lawless president wouldn’t continue to ignore the rule of law. However, it was becoming increasingly clear he was punting this issue down the road. Today’s unfortunate decision sends a very chilling signal to others who want to build infrastructure in this country. Roads, bridges, transmission lines, pipelines, wind farms and water lines will be very difficult, if not impossible, to build when criminal behavior is rewarded this way," North Dakota Representative Kevin Cramer said in a statement.

Mark Ruffalo, one of the celebrities bringing attention to the plight of the pipeline protesters, even visiting the site at least once, posted a ce;lebratory statement on Instagram, saying in part, "we can rejoice today because together we created greater justice in the world."

The news comes on a day when at least 2,000 U.S. military veterans have arrived at Standing Rock amid frigid cold to help battle against the construction of the pipeline.

The vets, led by Wesley Clark Jr., son of retired general and former presidential candidate Wesley Clark, began arriving in force today to help protest against the controversial crude oil pipeline project in North Dakota.

They joined the months-long demonstration at what felt like a moment of heightened drama: The North Dakota governor had issued an emergency evacuation order for protesters around the site, which follows a U.S. Army Corps of Engineers deadline for demonstrators to leave the area by Monday, Dec. 5.

But the evacuation order, which could have come with mass arrests, was made prior to today's statement by the Army.

Protesters and their supporters showed little inclination to back down, prior to the announcement this afternoon.

Donations to a GoFundMe account launched by Clark in support of Veterans for Standing Rock, a group he claimed would "assemble as a peaceful, unarmed militia at the Standing Rock Indian Reservation," passed the $1 million dollar mark this morning, coming from more than 24,000 individual donors, according to a page promoting the cause.

Standing Rock protesters described the veterans' mission as serving as a kind of "human shield" between peaceful demonstrators and police.

In addition to Clark's "peaceful militia," the U.S. Commission on Civil Rights told ABC News on Friday that it would send commissioners to North Dakota to monitor for any possible civil rights violations, as clashes between protesters and law enforcement have at times turned violent.

Kelcy Warren, CEO of Energy Transfer Partners, the Texas-based company behind the Dakota Pipeline, has argued that concerns about its potential to pollute water are unfounded.

He also wrote in an internal memo to staff in September that "multiple archaeological studies conducted with state historic preservation offices found no sacred items along the route," suggesting that the construction of the pipeline would not affect Native Americans who live in the area where it is being built.

ABC News' Evan Simon contributed to this report.