The Supreme Court has struck down a federal law barring the recognition of same-sex marriage in a split decision, ruling that the law violates the rights of gays and lesbians and intrudes into states' rights to define and regulate marriage. The court also dismissed a challenge to California's gay marriage ban, ruling that supporters of the ban did not have the legal standing, or right, to appeal a lower court's decision striking down Proposition 8 and clearing the way for gay marriage to again be legal in the nation's most populous state.

Justice Anthony Kennedy, the court's conservative-leaning swing vote with a legal history of supporting gay rights, joined his liberal colleagues in both decisions, which will dramatically expand the rights of married gay couples in the country to access more than 1,000 federal benefits and responsibilities of marriage previously denied them.

"The avowed purpose and practical effect of the law here in question are to impose a disadvantage, a separate status, and so a stigma upon all who enter into same-sex marriages made lawful by the unquestioned authority of the States," Kennedy wrote of DOMA. He concluded that states must be allowed by the federal government to confer "dignity" on same-sex couples if they choose to legalize gay marriage.

DOMA "undermines" same-sex marriages in visible ways and "tells those couples, and all the world, that their otherwise valid marriages are unworthy of federal recognition."

The case is Windsor v. United States, a challenge to the federal 1996 Defense of Marriage Act (DOMA), which prohibits the federal government from recognizing same-sex marriages even in the 12 states and District of Columbia that allow them. DOMA extended to more than 1,000 federal laws and statutes, including immigration, taxes, and Social Security benefits.

Eighty-three-year-old New Yorker Edith Windsor brought the suit after she was made to pay more than $363,000 in estate taxes when her same-sex spouse died. If the federal government had recognized her marriage, Windsor would not have owed the sum. She argued that the government has no rational reason to exclude her marriage of more than four decades from the benefits and obligations other married couples receive.

DOMA was signed into law by President Bill Clinton in 1996.

With this decision, Kennedy furthers his reputation as a champion of gay rights. He authored two of the most important Supreme Court decisions involving, and ultimately affirming, gay rights: Lawrence v. Texas (2003) and Romer v. Evans (1996). In Romer, Kennedy struck down Colorado's constitutional amendment banning localities from passing anti-discrimination laws protecting gays and lesbians. In Lawrence, Kennedy invalidated state anti-sodomy laws, ruling that gay people have a right to engage in sexual behavior in their own homes.

The decisions mark the first time the highest court has waded into the issue of same-sex marriage. Just 40 years ago, the Supreme Court tersely refused to hear a case brought by a gay couple who wanted to get married in Minnesota, writing that that their claim raised no significant legal issue. At the time, legal opinions often treated homosexuality as criminal, sexually deviant behavior rather than involuntary sexual orientation. Since then, public opinion has changed dramatically on gay people and same-sex marriage, with a majority of Americans only just recently saying they support it. Now, 12 states representing about 18 percent of the U.S. population allow same-sex marriage. With California, the percentage of people living in gay marriage states shoots up to 30.

The Supreme Court has refused to wade into the constitutional issues surrounding the California gay marriage case, dismissing the Proposition 8 argument on procedural grounds. The legal dodge means a lower court's ruling making same-sex marriage legal in California will most likely stand, opening the door to marriage to gays and lesbians in the country's most populous state.

California voters passed Proposition 8 to ban same-sex marriage in 2008, after 18,000 same-sex couples had already tied the knot under a state Supreme Court decision legalizing gay marriage. A same-sex married couple with children, Kris Perry and Sandy Stier, sued the state of California when their six-month-old marriage was invalidated by the ballot initiative. They argued that Proposition 8 discriminated against them and their union based only on their sexual orientation, and that the state had no rational reason for denying them the right to marry. Two lower courts ruled in their favor, and then-California Gov. Arnold Schwarzenegger announced he would no longer defend Proposition 8 in court, leaving a coalition of Prop 8 supporters led by a former state legislator to take up its defense.

Same-sex marriage will most likely not be immediately legal in California, since the losing side is given a few weeks to petition the courts.

[Live coverage of the gay marriage decisions.]

The Prop 8 case was argued by two high-profile lawyers, Ted Olson and David Boies, who previously faced off against each other in Bush v. Gore. Olson, a conservative and Bush's former solicitor general, and Boies, a liberal, have cast gay marriage as the civil rights issue of our time.

Boies said on the steps of the Supreme Court Wednesday that "Today the United States Supreme Court said as much. They cannot point to anything that harms them because these two love each other”

Olson made the argument that gay marriage should be a conservative cause in a recent interview with NPR. "If you are a conservative, how could you be against a relationship in which people who love one another want to publicly state their vows ... and engage in a household in which they are committed to one another and become part of the community and accepted like other people?"

The Bipartisan Legal Advisory Group (BLAG), a coalition of mostly Republican House lawmakers, defended DOMA since the Obama administration announced they believed the law was unconstitutional in 2011. (Chief Justice John Roberts criticized the president for this move during oral arguments in the case, saying the president lacked “the courage of his convictions” in continuing to enforce the law but no longer defending it in court.)