Former US senator and Rhode Island governor Lincoln Chafee will seek the nomination of the Democratic party to run for US president in 2016.
“We must deliberately and carefully extricate ourselves from expensive wars,” Mr Chafee said on Wednesday.
Mr Chafee has served as a Republican and an Independent but has never before run for office as a Democrat.
Analysts say he will aggressively attack presumptive front-runner Hillary Clinton on her foreign policy record.
In his announcement speech, he called for the US to move to the metric system and end capital punishment. He also advocated an “open-minded approach” to drug trafficking.
But his priority would be to end all US wars.
“Let’s wage peace in this new American century,” he said.
Mr Chafee joins Mrs Clinton, Vermont Senator Bernie Sanders and former Maryland Governor Martin O’Malley in the Democratic primary race.
Analysis: Anthony Zurcher, Washington
There are candidates who have a challenging road to their party’s presidential nomination. There are candidates who have near insurmountable objects blocking their path to the prize – usually money and campaign organisation.
And then there’s Lincoln Chafee.
The Rhode Island Republican-turned-independent-turned-Democrat raised eyebrows in April when he hinted, with little apparent warning or visible preparation, that he was considering a Quixotic bid for the Democratic nomination.
But here he is, announcing that he will be the third candidate to challenge prohibitive favourite Hillary Clinton for the party’s presidential nod.
It’s difficult to imagine a winning path for the second-generation US senator, whose biggest headline on the day he entered the race was that he had lost the login information for the Facebook page he maintained while running for governor.
Politico’s Ben White gave it a try, however, tweeting his suggested campaign slogan: “Chafee 2016: Because all the other candidates might die.”
Mr Chafee has so far been the Democrat most willing to directly criticise Ms Clinton, particularly over her Senate vote in favour of the Iraq War.
“I don’t think anybody should be president of the United States that made that mistake,” Mr Chafee told the Washington Post in April.
Mr Chafee’s rivals have already built robust campaign operations with staff across the country, a step he has yet to take.
Before splitting with the Republicans he had consistently been called the party’s most liberal lawmaker.
In 2002 he was the only Republican member of the Senate to vote against authorising the invasion of Iraq.