McCain discussed Iraq with British Prime Minister Gordon Brown in their first-ever meeting - a 45-minute discussion in London that also touched on the economy and climate change, McCain said.
McCain's political fortunes are linked to the war he strongly backs and the four-term Arizona senator said Iraq would likely be the defining issue of the election. McCain has clinched enough delegate support to win the Republican Party's nomination.
"We are now succeeding in Iraq," the Arizona Senator told a gathering of journalists outside Gordon Brown's official residence at No. 10 Downing Street.
But, CBS News correspondent Sheila MacVicar reports, McCain had to dance around questions about the U.K.'s rapidly diminishing military contribution to the war. Britain has about 4,000 troops based at an airport camp outside Iraq's southern city of Basra, but hopes to cut the number to 2,500 within months.
"I believe that decision is made by the British government and people. We appreciate, enormously the long service and sacrifice of the British men and women in the military," McCain said diplomatically.
McCain said this week, however, that pulling out of Iraq quickly would be a mistake that would boost Iran and al Qaeda. On Thursday, he said Americans were increasingly backing the U.S. troop increase strategy and believed the tactic was bringing success.
When pressed, McCain said he had "issued those warnings concerning the United States of America."
"That will be, frankly, a very big issue for the country, whether we withdraw and have al Qaeda win and announce to the world they have won and have things collapse there, or whether we see this strategy through to success," McCain said.
Brown has stressed that British troops would be withdrawn only on the advice of defense chiefs. Some officials suggest fragile security in Basra may force the U.K. to delay any pullout.
"We appreciate enormously the long service and sacrifice of the British men and women in the military both in Iraq and Afghanistan," McCain told reporters outside Brown's official residence in London.
More than one reporter asked him to answer criticism coming from his rivals that, if elected, he would continue President Bush's strategy in Iraq. "Americans, in significant numbers, agree that the present strategy of the surge is succeeding," McCain responded.
"Having just come from Iraq I can tell you, unequivocally, that the situation has improved dramatically over the last year. People are going about their normal lives. But the fact is al Qaeda is on the run, they are not defeated. They are not defeated, and we're going to have to continue training the Iraqi military and doing what we've been doing if we're going to achieve a stable situation in Iraq."
McCain's visit to Britain was part of a weeklong tour of Middle East and Europe. He was traveling as part of a U.S. congressional delegation - including Sens. Joe Lieberman, I-Conn., and Lindsey Graham, R-S.C. - that already has visited Iraq, Jordan and Israel.
The McCain campaign has defended the trip - at taxpayers' expense - as critical for members of the Senate Armed Services Committee. However, McCain was using some of his time overseas to collect money for his White House bid. He had a $1,000 per person fundraising lunch at London's Spencer House.
"Having just come from Iraq, I can tell you unequivocally that the situation has improved dramatically over the last year," McCain said. "Al Qaeda is on the run. They are not defeated."
But he acknowledged some mistakes had been made in postwar planning. "The problem with Iraq, in my view, is because it was mishandled after the initial success, that caused great frustration, sacrifice and sorrow on the part of the American people and our allies," he said.
McCain also met Thursday with Britain's opposition Conservative party leader David Cameron at the Houses of Parliament, where a host of Conservative lawmakers lined a courtyard beneath the Big Ben clock tower to greet McCain as he arrived.
"Our conversation centered on Afghanistan," Cameron said. "The senator and I also spent time discussing our shared interests and how we can broaden the appeal of moderate Conservatism."
McCain also planned a meeting with Europe's environment commissioner, Stavros Dimas, to discuss U.S. climate change policy, European Commission spokeswoman Emilia Hinkkanen said.
Dimas is attempting to win support from the three major presidential candidates for a plan to establish carbon trading in the U.S. to help cut carbon emissions.
"I am convinced that if we work at it, we will be able to convince India and China that it is in their interest to be part of a global agreement to reduce greenhouse gas emissions," McCain said after meeting Brown. "I believe that we can achieve a global agreement."
He planned a trip to Paris on Friday to meet with French President Nicolas Sarkozy, whom he called a leader with "pro-American tendencies."