Barack Obama has reaffirmed the special relationship between the US and the UK and declared that together the two countries "stand squarely on the side of those who long to be free".
Rounding off his state visit to Britain, the president rejected the argument that the rise of new economic powers such as China had sidelined Europe and America, insisting that "the time for our leadership is now".
At a "pivotal moment" in history, with demands for democracy across the Arab world and an international coalition fighting oppression in Libya, Britain and America remained "indispensable to the goal of a century which is more peaceful, more prosperous and more just", he said.
Mr Obama was speaking to both Houses of Parliament in the historic Westminster Hall shortly after talks with Prime Minister David Cameron at 10 Downing Street.
In a joint press conference, the two leaders agreed that the transatlantic special relationship was now an "essential relationship" for global stability and prosperity.
Insisting there would be "no let-up" for Muammar Gaddafi until he stood aside and allowed the Libyan people to shape their own future, they pledged to "turn up the heat" on Tripoli and other brutal authoritarian regimes.
But they stressed they had "learned the lessons" from military adventures under Tony Blair and George Bush, and were taking action in Libya as part of a wide coalition operating within the terms of a clear UN mandate.
Rather than Blair and Bush, it was Roosevelt and Churchill that Mr Obama evoked as he praised the virtues of the special relationship in his keynote speech at Westminster Hall.
Speaking to an audience including Mr Cameron and former PMs Gordon Brown, Mr Blair and Sir John Major, Mr Obama said he had known "few greater honours" than becoming the first US President to address what he termed "the mother of Parliaments" in the 900-year-old hall.
It is an honour previously granted to only a handful of eminent figures including Nelson Mandela, the Queen and Pope Benedict XVI - a line-up who Mr Obama quipped represented "either a very high bar or the beginning of a very funny joke".