May 1996: Overwhelming majorities in both houses of Congress vote for the Defense of Marriage Act, defining marriage at a federal level as between one man and one woman. In September, President Bill Clinton signs the bill into law.
March 2000: California voters, by 61 percent to 39 percent, pass Proposition 22, which defines marriage in state law as a union between a man and a woman. Then-Sen. William J. “Pete” Knight was the official sponsor of the initiative.
April 2000: Vermont Gov. Howard Dean signs a bill allowing same-sex couples to form civil unions, a first in the country.
November 2003: The Massachusetts Supreme Court strikes down the state’s ban on gay marriage as discriminatory. The state begins issuing marriage licenses to same-sex couples the following May.
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February 2004: Declaring that the California Constitution prohibits discrimination on the basis of sexual orientation, San Francisco Mayor Gavin Newsom orders the city clerk to grant marriage licenses to same-sex couples. Hundreds of couples flock to the city to get married. A month later, responding to a lawsuit filed by then-Attorney General Bill Lockyer, the California Supreme Court orders a temporary halt to the marriages while it considers the case.
February 2004: President George W. Bush calls for a constitutional amendment defining marriage between one man and one woman. That November, he wins re-election, and voters in 11 states pass same-sex marriage bans.
August 2004: The California Supreme Court rules that Newsom had no legal authority to order the issuance of marriage licenses to same-sex couples and strikes down those marriages as invalid. The court, however, leaves the door open to a legal challenge on the constitutionality of Proposition 22.
September 2005: The Legislature passes AB 849, by then-Assemblyman Mark Leno, in an attempt to legalize same-sex marriages in California. Gov. Arnold Schwarzenegger vetoes the bill, saying it would violate the will of the people as expressed in the passage of Proposition 22. He says only the courts or the people can overturn that ban. Two years later, he again vetoes a similar bill.
May 2008: The state Supreme Court rules 4-3 that Proposition 22’s statutory ban on gay marriage violates the state constitution. The decision, written by Chief Justice Ronald George, legalizes same-sex marriages in California, which begin a month later.
November 2008: California voters, by 52 percent to 48 percent, pass Proposition 8, which places into the state constitution the definition of marriage as between one man and one woman.
April 2009: Vermont becomes the first state to legalize gay marriage through the Legislature, rather than a court decision, overriding a veto by Gov. Jim Douglas.
August 2010: U.S. District Judge Vaughn Walker declares that California’s Proposition 8 violates gay people’s constitutional equal-protection and due process rights. His decision works its way up to the U.S. Supreme Court, which agrees in December 2012 to hear the case.
November 2012: Voters in Maine, Maryland and Washington state approve ballot measures to authorize same-sex marriages, the first time it has been done by popular vote.
June 2013: In a 5-4 decision, the U.S. Supreme Court rules that gay-marriage opponents did not have standing to appeal a lower court ruling that overturned the Proposition 8 ban on gay marriage. In a separate 5-4 decision, the court also strikes down key parts of the Defense of Marriage Act, which blocked gay married couples from receiving a host of federal benefits. Days later, same-sex marriages resume in California.
November 2014: Following a series of federal appeals court decisions clearing the way for gay marriage in dozens of states, the 6th U.S. Circuit Court of Appeals rules to uphold bans in Michigan, Ohio, Kentucky and Tennessee. This decision ultimately makes its way to the Supreme Court.
June 2015: In a 5-4 decision, the U.S. Supreme Court declares gay marriage bans unconstitutional, allowing same-sex couples nationwide to wed. States must also recognize marriages performed elsewhere.