It was covered with a huge scaffold, to make it look like a stockpile of Bamboo and misguide any enemy bombers. The camouflaging process is still incomplete in this photo. It is said the whole of Taj Mahal was covered, but this picture shows only the main dome covered. Maybe the British govt didn’t allow any photographers later to shoot the final scaffold cover.
During the India-Pakistan war in 1971, it was protected by covering it with a green cloth and making it almost invisible i.e camouflaged within the greenery around it. Even in 2001, after the Sep 11 attack, Archaeological Survey of India took up the precautionary measure to cover it with cloth and it took them more than 20 days to do that.
However these days with the advent of high precision GPS, and satellite imagery an enemy can bomb such precious targets blindly. But the Taj Mahal has actually never been a target of our enemies but rather sadly the thousands of ignorant people who visit the site everyday and damage the structure by touching, scratching and making loud noise. More damage is being done by thousands of industries who freely pollute the Yamuna and the air around Taj Mahal. There have been serious allegations of corruption in the pollution control board.
The archaeological survey of India too needs to wake up and do much more in preserving such a beautiful monument of glorious Indian heritage. Shah Jahan would definitely shed tears if he sees his masterpiece in such a sad state.