Complex sentence: By 1960, one-sixth of working Americans were employed directly or indirectly by the industry, but as automation and imports eroded the need for such a large workforce within a couple of decades.
Split into simpler sentences: By 1960, one-sixth of working Americans were employed directly or indirectly by the industry. This massive work force was short-lived, however, as automation and imports eroded the need for such a large workforce within a couple of decades.