Former president Bill Clinton and British Prime Minister Tony Blair in 2006. Three years earlier, Blair asked Clinton to help round up support for a U.N. resolution aimed at avoiding the Iraq War. (J. Emilio Flores/Getty Images)

It was March 2003, and British Prime Minister Tony Blair had summoned his friend Bill Clinton to Chequers Court, his country home in England, to make an urgent request: Could the former president quietly help corral U.N. Security Council members to back a resolution aimed at slowing or, according to Blair, even stopping the Iraq War?

The events that followed show Clinton taking an unprecedented and un­or­tho­dox role in the foreign policy of his successor, George W. Bush, according to a new book, “Man of the World: The Further Endeavors of Bill Clinton,” written by liberal journalist Joe Conason.

Clinton’s activism came months after his wife’s vote in October 2002 in favor of authorizing Bush to use military force in Iraq — a decision that, according to the book, Bill Clinton counseled her to make. And the former president acted without the express approval of the White House.

The 13-year-old incident highlights the 42nd president’s outsize role in foreign affairs even after he left Washington. It is an exception to the pattern of Clinton’s post-presidential diplomatic work, which was usually done at the request of his successors — such as his trip to North Korea in 2009 to help rescue imprisoned journalists at the request of President Obama.

Former presidents George W. Bush and Bill Clinton at Bush’s presidential library in Dallas in 2015. A spokesman for Clinton said his 2003 efforts were in keeping with the Bush administration’s public position at the time. (Mike Stone/Reuters)

In Conason’s account, Clinton made last-minute appeals to several world leaders he considered “friends,” asking them to back Blair’s resolution in the hopes of slowing Bush’s march to war or ending it altogether.

Blair’s resolution would have set a three-week timeline for U.N. weapons inspections teams led by diplomat Hans Blix to complete their work. If Iraqi President Saddam Hussein interfered with the inspections or refused to disarm, the resolution would have authorized force.

During a speech in Washington after meeting with Blair, Clinton voiced support for the approach outlined in the resolution. And privately, he began a complementary effort to tip the scales in favor of the resolution at the Security Council, according to Conason’s book.

“Privately, Clinton arranged a discreet contact with Chilean president [Ricardo] Lagos through a back channel arranged by his former White House chief of staff, Thomas ‘Mack’ McLarty, who was acquainted with the Chilean interior minister, José Miguel Insulza,” Conason writes. “Not wishing to appear to intervene in matters between heads of state, Clinton asked McLarty to pass a message to Lagos via Insulza.”

In the message, Clinton told the Chilean minister that the resolution was “a last chance to prevent war,” according to Conason.

The Chileans would get on board only if the Mexicans did. So Clinton phoned Vicente Fox, then president of Mexico, to lobby for his support, the book says.

“Fox and other Mexican officials contacted by Clinton told him that they were wary of any resolution that might somehow be interpreted as supporting a war that nearly everyone in their country opposed,” Conason wrote. “If a resolution passed that gave Blix three more weeks, and then he came back and asked for additional time, they asked, wouldn’t Bush invade anyway?”

Speaking to reporters during a campaign stop in Iowa, May 19, 2015, Democratic presidential candidate Hillary Clinton said she made a mistake when she voted to authorize the Iraq war when she was a senator representing New York. (Reuters)

Ultimately, the Chileans and the Mexicans were loath to support a resolution that included a threat of force, which they believed would ultimately lead to war. The Chileans would later unveil their own resolution to give inspections more time, but it lacked the threat of force. That resolution was sharply rejected by the United States.

A spokeswoman for Blair declined to comment.

In Blair’s response to a British inquiry into the run-up to the war — the probe, completed in July, was sharply critical of the prime minister’s actions — he noted that the United States had agreed to a resolution with new tests for Hussein that might have avoided war. But, he said, the United States “understandably insisted that in the event of continued failure, the UN had to be clear that action would follow.”

In a letter to Bush in late February 2003, Blair wrote that the resolution could help gain the backing of European public opinion if war came. “It allows us to show the world that we are going to war, not because we want to, but because we have to,” Blair wrote.

Blair’s resolution was a failure, and the war began on March 20, 2003.

Conason describes Clinton’s gambit as “risky” and contrary to the norms against former presidents criticizing or interfering with the administration of a sitting president.

A spokesman for Clinton said his efforts were in keeping with the Bush administration’s public position at the time.

“President Clinton tried to expand support for a resolution that would’ve allowed more time for the inspectors to complete their investigation,” said Angel Urena, Clinton’s spokesman. “His efforts were consistent with the administration’s stated policy at the time. To suggest otherwise would be inaccurate.”

Conason writes that the episode was a break from Clinton’s usual habit after leaving the White House of informing the State Department or the National Security Council when he traveled overseas or met with heads of state.

“As a matter of post-presidential formality, it definitely broke the rules,” Conason said in an interview. “As a matter of human morality, it was a good idea to try to stop [the war].”

According to Conason, Bush may have learned about Clinton’s actions as they were happening or afterward. The author even suggests that the National Security Agency, which was spying on U.N. Security Council members, was likely to have picked up on the conversations.

Clinton “wanted to make sure the inspections were completed,” Conason told The Washington Post. “And he also believed that if they found no weapons, which was likely, that the invasion would not go forward and they would have to reconsider what they were doing.”

It is unclear how Clinton’s actions affected official U.S. efforts to bring Chile and Mexico in line with Washington’s goals. But Conason notes that Chile’s president, Lagos, was “prodded by his conversation with Clinton” to proceed with drafting the separate resolution that was later rejected by the Bush administration.

Years later, Heraldo Muñoz, a top Chilean diplomat, wrote a book in which he said the diplomatic strong-arming of Latin American countries by the Bush administration over the war damaged U.S. relations with those countries.

Hillary Clinton has called the authorization vote for the war “probably the hardest decision I’ve ever had to make,” and it has haunted her political career.

Bill Clinton advised his wife that the “politically smart vote was no, but if she believed President Bush was serious about letting the inspections proceed, a credible threat of force would greatly increase the chance of Saddam’s cooperation,” a Clinton aide wrote in an email to The Post. The aide spoke on the condition of anonymity to discuss the private conversations between the Clintons.

First the Democratic Party and then America as a whole would turn against the war, leaving Hillary Clinton apologetic and regretful. Her vote became a centerpiece of Obama’s successful presidential campaign against her in 2008. The issue arose again in the 2016 Democratic primary race against Sen. Bernie Sanders (Vt.), who voted against the war when he was a congressman.

“I came to deeply regret giving President Bush the benefit of the doubt on that vote,” Clinton wrote years later in her memoir “Hard Choices.”