It was 1973 when I crossed the hundred-step bridge into the Middle Kingdom. It felt like a John Le Carré spy novel. Ahead, a baggy-green-uniformed Chinese soldier hugged an AK-47. Above him loomed a huge red poster emblazoned with the words of demi-god, Mao: 'We Have Friends All Over the World'. I wondered if that was a welcome, a warning, or both