Complex sentence: Valentin J. Peter, editor of the German-language "Omaha Tribune", was chiefly responsible for the formation and leadership of the National German-American Alliance, which exerted tremendous influence of the German-American electorate across the United States between 1912 and 1940.
Split into simpler sentences: Valentin J. Peter, editor of the German-language "Omaha Tribune", was chiefly responsible for the formation and leadership of the National German-American Alliance. This coalition was influential in representing and leading the German-American electorate across the United States between 1912 and 1940.