(From L - R) US President Barack Obama stands alongside former presidents George W. Bush, Bill Clinton, George H.W. Bush and Jimmy Carter as they attend the dedication ceremony for the George W. Bush Presidential Center in Dallas, April 25, 2013. Obama is in Texas to stand shoulder-to-shoulder with former President George W. Bush in what could serve as a powerful reminder of the ongoing struggle against terrorism, from the Sept 11 attacks to the Boston Marathon bombings.[Photo/Agencies]