In a sternly worded column in The Times of London, the two leaders liken the effort to free Arab peoples from authoritarianism to the Cold War and the fall of the Berlin Wall in the 1980s.
They also liken their personal efforts to two leaders who came before them: Ronald Reagan and Margaret Thatcher.
"We will stand with those who want to bring light into dark, support those who seek freedom in place of repression, aid those laying the building blocks of democracy," they wrote.
"We will not stand by as their aspirations get crushed in a hail of bombs, bullets and mortar fire. We are reluctant to use force, but when our interests and values come together, we know that we have a responsibility to act."
The leaders pledged to finish the military effort in Libya and stick together in Afghanistan. They did not mention other hot spots where intervention may prove necessary, such as Syria, or where human rights are being denied, such as Saudi Arabia -- but their sentiments were implicit.
Obama and Cameron will meet today, but their formal bilateral session comes on Wednesday, when they also hold a press conference and Obama addresses both houses of Parliament. Today is reserved mostly for formalities associated with the president's State visit, including a dinner hosted by Queen Elizabeth II.
In their column, they recast the "special relationship" enjoyed by the two nations as an "essential relationship -- for us and for the world."
"When the United States and Britain stand together, our people and people around the world can become more secure and more prosperous," they wrote.