Complex sentence: In 1966, Carto acquired control of "The American Mercury", a magazine which was once a highly respected periodical associated with H.L. Mencken, but which was failing by the time Carto acquired it.
Split into simpler sentences: In 1966, Carto acquired control of "The American Mercury". The magazine was once a highly respected periodical associated with H.L. Mencken, but which was failing by the time Carto acquired it.