China has a love-hate relationship with what is foreign. Traditionally all people beyond the Great Wall were barbarians - only part human. But invaders have sometimes been welcomed, in time, into the Chinese family. One was Kublai Khan. In the 13th Century, no-one knew how big the world was so it was not so wild for the Mongols to set off from the grassland with the idea that they were going to conquer all of it. When the mighty Genghis Khan died in 1227, he had already claimed an empire stretching from the Pacific to Europe. His grandson Kublai set out to finish the job - and started by moving south to attack China's Song dynasty. But China had been a united empire on and off for more than 1,000 years. So what did the Song dynasty rulers make of Kublai's ambition? Presenter: Carrie Gracie Producer: Neal Razzell.