The strike, called in September 1913 by the United Mine Workers, over the issue of union representation, was against coal mine operators in Huerfano and Las Animas counties of southern Colorado, where the majority of CF&I's coal and coke production was located. The strike was fought vigorously by the coal mine operators association and its steering committee, which included Welborn, president of CF&I, a spokesman for the coal operators. Rockefeller's operative, Lamont Montgomery Bowers, remained in the background. Few miners actually belonged to the union or participated in the strike call, but the majority honored it. Strikebreakers (called "scabs") were threatened and sometimes attacked. Both sides purchased substantial arms and ammunition. Striking miners were forced to abandon their homes in company towns and lived in tent cities erected by the union, such as the tent city at Ludlow, a railway stop north of Trinidad.
Rockefeller created the Rockefeller Foundation in 1913 to continue and expand the scope of the work of the Sanitary Commission, which was closed in 1915.
His wealth continued to grow significantly (in line with U.S. economic growth) as the demand for gasoline soared, eventually reaching about $900 million on the eve of the First World War, including significant interests in banking, shipping, mining, railroads, and other industries. His personal wealth was 900 million in 1913 worth 21 billion dollars adjusted for inflation in 2016. According to his New York Times obituary, "it was estimated after Mr. Rockefeller retired from business that he had accumulated close to $1,500,000,000 out of the earnings of the Standard Oil trust and out of his other investments. This was probably the greatest amount of wealth that any private citizen had ever been able to accumulate by his own efforts." By the time of his death in 1937, Rockefeller's remaining fortune, largely tied up in permanent family trusts, was estimated at $1.4 billion, while the total national GDP was $92 billion. According to some methods of wealth calculation, Rockefeller's net worth over the last decades of his life would easily place him as the wealthiest known person in recent history. As a percentage of the United States' GDP, no other American fortune—including those of Bill Gates or Sam Walton—would even come close.