Armed policemen stand guard at the gates

Armed policemen stand guard at the gates of Downing Street in London on January 23, 2010. Britain raised its assessment of the terror threat from substantial to severe, the second highest level, suggesting an attack on the country is "highly likely", Home Secretary Alan Johnson said. The change was announced just weeks after a failed plane bombing in the United States, and days ahead of two major international conferences on Yemen and Afghanistan in London. AFP PHOTO/Ben Stansall (Photo credit should read BEN STANSALL/AFP via Getty Images)