Kimberly Rivera complied with a deportation order and presented herself at the border at Gananoque, Ont., on Thursday.
The War Resisters Support Campaign — which issued multiple warnings that Rivera would likely face a court martial and jail time on her return — said the mother of four was immediately arrested, detained and transferred to U.S. military custody.
"Kimberly now awaits punishment for refusing to return to Iraq, a conflict which Kimberly and Canada determined was wrong," the group said in a statement.
Rivera was being held in Fort Drum, N.Y., some 360 kilometres from Toronto, and was waiting to be transferred to a different military facility where she faces punishment for being absent from her unit, the group said.
The 30-year-old's husband and her children — two of whom were born in Canada — crossed the border separately on Thursday.
"She didn't want her children to see her arrested by the military," said spokesman Ken Marciniec.
The parliamentary secretary to Immigration Minister Jason Kenney confirmed the deportation in the House of Commons, drawing a huge cheer from the Conservative benches.
"Our government does not believe that the administration of the president or the president himself in any way, shape, or form, is going to persecute Ms. Rivera," Rick Dykstra said.
"In fact, she has had every opportunity in this country, despite the fact that not one of the applications from an American war deserter has been successful in Canada. Each and every one of them has been upheld by the Federal Court in terms of the Immigration and Refugee Board denying them."
Rivera's supporters — who had been hoping for a last-minute intervention from the government — said the Army private's case clearly demonstrated that conscientious objectors to the Iraq war are targeted for punishment.
"It doesn't get any clearer that this," Marciniec said. "The risk that we've pointed out, of Iraq War resisters being punished as prisoners of conscience, isn't just risk. It's fact. Kim's case today proves that."
Marciniec also pointed out that two other Iraq war resisters who were deported in the past — Robin Long and Clifford Cornell — faced year-long jail sentences for desertion upon their return.
Rivera, who lived in Toronto with her family, came to Canada in 2007 to avoid further U.S. military service.
She has said she grew to oppose the Iraq war while she was taking part in it, and even stopped carrying her rifle with her.
She told reporters last month that her biggest fear about being deported was being separated from her young children and having to sit in a prison for politically being against the Iraqi conflict.
Rivera received her deportation order after a negative pre-removal risk assessment. That assessment ruled she would not be in danger of punishment, torture or loss of life if deported.
Her lawyers had said the ruling didn't take into account Rivera's outspoken objection to the Iraq war — something they said other publicly critical soldiers have been penalized for.
Rivera had applied for Canadian permanent residency on humanitarian and compassionate grounds, but a decision on her application is pending.
On Monday, a Federal Court judge denied her request for a stay of removal, finding the possibility of her arrest and detention in the U.S. to be speculative.
Some 20,000 people signed an online petition protesting Rivera's deportation order and rallies were held in a number of Canadian cities Wednesday calling on the government to let her stay in the country.
Nobel Peace Prize winner Archbishop Desmond Tutu and the American veterans organization Veterans for Peace have also spoken out against the deportation.
Rivera's deportation was called an "international tragedy" by the United Steeworkers on Thursday.
"This gives Canada a black eye on the international stage. Our country’s once-proud tradition as a safe haven for conscientious objectors has been destroyed with Kim’s deportation," national director Ken Neumann said.
"Immigration Minister Jason Kenney had the opportunity to show compassion and do the right thing, and he refused to act."
During the Vietnam War, up to 90,000 Americans won refuge in Canada, many of them to avoid the military draft. Many were given permanent residence status that led to Canadian citizenship.
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