WASHINGTON (CNN) -- In a dramatic slap at federal authority, a divided Supreme Court has struck down a key part of congressional law that denies to legally married same-sex couples the same benefits provided to heterosexual spouses.

The Defense of Marriage Act defines marriage as only between a man and a woman.

The vote Wednesday was 5-4. [RELATED: High court dismisses California’s Prop. 8 appeal]

"Although Congress has great authority to design laws to fit its own conception of sound national policy, it cannot deny the liberty protected by the Due Process Clause of the Fifth Amendment," said Justice Anthony Kennedy. [BONUS: Click here to read the high court's 77-page ruling on DOMA]

The case examined whether the federal government can deny tax, health and pension benefits to same-sex couples in states where they can legally marry. At issue was whether DOMA violates equal protection guarantees in the Fifth Amendment's due process clause as applied to same-sex couples legally married under the laws of their states.

The key plaintiff is Edith "Edie" Windsor, 84, who married fellow New York resident Thea Spyer in Canada in 2007, about 40 years into their relationship. By the time Spyer died in 2009, New York courts recognized same-sex marriages performed in other countries. But the federal government didn't recognize Windsor's same-sex marriage, and she was forced to assume an estate tax bill much larger than those that other married couples would have to pay. So, Windsor sued the federal government.

.@JanCBS: This decision means some laws that treat same sex couples differently are going to be difficult to justify. #SCOTUS #DOMA — CBS News (@CBSNews) June 26, 2013

A federal appeals court last year ruled in Windsor's favor, saying DOMA violated the Constitution's equal protection clause.

CNN Supreme Court producer Bill Mears writes that Defense of Marriage Act (DOMA), passed in 1996, defines marriage as between one man and one woman for federal purposes, like taxes. "That means the estimated 120,000 gay and lesbian couples legally married in nine states and the District of Columbia are still considered -- in the eyes of DOMA opponents -- the equivalent of girlfriend and boyfriend."

That meant that Edie Windsor faced a hefty bill for inheritance taxes when her partner of 42 years died. She claimed in court that she had had to pay $363,053 more than if her spouse, Thea Spyer, had been a man.

But Mears points out that the DOMA issue is more than just a financial question:

"The larger debate over DOMA's intent and impact 17 years after passage has driven a wedge between the executive and legislative branches.

"At issue is what role the federal government should play when it comes to marriage - something states have traditionally controlled."

The other key case expected to be decided today considers Proposition 8. "In the 'Prop 8' case, the high court is being asked to establish a constitutional 'equal protection' right. It is the kind of hot-button issue that will define our society, our laws, our views on family," Mears writes.