What is the Japanese "Fifth Column" that was used in World War II that everyone was so afraid of?

Emilio Mola, a Nationalist Genral during the Spanish Civil War, told a journalist in 1936 that as his four columns of troops approached Madrid, a "fifth column" of supporters inside the city would support him and undermine the Republican government from within. The term was then widely used in Spain. Ernest Hemingway used it as the title of his only play, which he wrote in Madrid while the city was being bombarded, and published in 1938 in his book The Fifth Column and the First Forty-Nine Stories[1] Some writers, mindful of the origin of the phrase, use it only in reference to military operations rather than the broader and less well defined range of activities that sympathizers might engage in to support an anticipated attack. Madeleine Albright for example, in a lengthy account of German sympathizers in Czechoslovakia in the first years of World War lI, reserves it for their possible response to a German invasion: "Many, perhaps most, of the Sudetens would have provided the enemy with a fifth column". [2]