U.S. President Barack Obama and British Prime Minister David Cameron on Tuesday hailed the " unique and essential” American-British alliance in dealing with world challenges, ahead of their summit meeting.Cameron began his three-day visit to the U.S. Tuesday, during which he will discuss with Obama a wide range of issues including how to deal with the global economy, Afghanistan’ s transition, Iranian nuclear standoff and Syria’s unrest.  In a joint article published on The Washington Post, Obama and Cameron said that, with the American-British alliance, "there is hardly anything they (Americans and British) could not do, either in the field of war or in the not less tangled problems of peace.”"The alliance between the United States and Great Britain is a partnership of the heart, bound by the history, traditions and values we share. But what makes our relationship special -- a unique andessential asset -- is that we join hands across so many endeavors,” they wrote."Put simply, we count on each other and the world counts on our alliance,” they said.As leading world economies, the two countries are coordinating closely with their G-8 and G-20 partners "to put people back to work, sustain the global recovery, stand with our European friends as they resolve their debt crisis and curb the reckless financial practices that have cost our taxpayers dearly,” the two leaders said.The two leaders also highlighted the unity and contributions of the two allies in the mission in Afghanistan, in dismantling al- Qaeda,breaking the Taliban’s momentum and training Afghan forces.