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A federal appeals court in New York today struck down a key provision of the Defense of Marriage Act, (DOMA) ruling that section 3 of the federal law that defines marriage as between a man and a woman violates the equal protection clause of the Constitution.

"DOMA's classification of same-sex spouses was not substantially related to an important government interest," Chief Judge Dennis Jacobs wrote for the 2-1 court, "Accordingly, we hold that section 3 of DOMA violates equal protection and is therefore unconstitutional."

The opinion marks the second time an appeals court has struck down the law, but the first time that an appeals court has relied upon a heightened scrutiny standard in doing so.

"Heightened scrutiny means that if the government is going to discriminate against a particular group, it needs to have a very convincing justification for the discrimination," said Tobias B. Wolff a professor of law at the University of Pennsylvania.

The New York case involves Edith Windsor, who in 2007 married Thea Spyer, her partner of more than 40 years. The couple were married in Canada, but resided in New York until Spyer died in 2009. Windsor was forced to pay $363,000 in federal estate taxes. She applied for a refund, believing she was entitled to a marital deduction, but she was denied the claim on the grounds that she was not a "spouse" within the meaning of DOMA.

"Yet again, a federal court has found that it is completely unfair to treat married same-sex couples as though they're legal strangers," James Esseks, director of the ACLU LGBT Project and a lawyer for Windsor, said after the ruling. "Edie and Thea were there for each other in sickness and in health like any other married couple, and it's unfair for the government to disregard both their marriage and the life they built together and treat them like second-class citizens."

In 2011 the Obama administration decided it could no longer defend DOMA in court, arguing that it was unconstitutional. In court briefs, Solicitor General Donald B. Verrilli Jr. described the impact of the law.

"Although section 3 of DOMA does not purport to invalidate same-sex marriages in those states that permit them, it excludes such marriage from recognition for purposes of more than 1,000 federal statutes and programs whose administration turns in part on individuals' marital status."

Because the government refused to defend the law in court, Speaker of the House John Boehner, R-Ohio, moved to intervene and appointed the U.S. House of Representatives Bipartisan Legal Advisory Group (BLAG) to do so.

Paul D. Clement serves as BLAG's lawyer and stressed that DOMA was enacted with strong majorities in both houses of Congress and was signed into law in 1996 by President Bill Clinton. In court briefs, Clement wrote that DOMA was not meant to invalidate any marriages, but "simply asserts the federal government's right as separate sovereign to provide its own definition which governs only federal programs and funding."

Today's decision was written by Chief Judge Dennis Jacobs, who is considered a conservative judge and was nominated to the bench by George H.W. Bush.