President William McKinley was assassinated in 1901, and Theodore Roosevelt became President. At the time, many people felt that large corporations were using unfair business practices. Writers, known as muckrakers, revealed dishonest business practices. Upton Sinclair wrote about the meat- packing industry, and Ida Tarbell wrote about Standard Oil. Meanwhile, Roosevelt worked to reform large corporations. He believed the government should be able to break up trusts, or businesses that fixed prices to get rid of competition.

In 1903, Roosevelt purchased from Panama a ten-mile wide strip of land. Shortly after, Roosevelt began making plans to build the Panama Canal. With a canal, ships would no longer have to travel around South America to reach the Caribbean Sea; it would cut 7,000 miles off such a trip. Building the canal was dangerous work, and many died from malaria and yellow fever.

In 1903, Orville and Wilbur Wright invented the first aircraft with an engine. Orville made the first flight on December 17, 1903. In 1913, Henry Ford invented the assembly line and used it and interchangeable parts to mass-produce automobiles. President Roosevelt invented ways to protect our wilderness areas. He set aside about 150 million acres of land for national parks and national monuments, such as the Grand Canyon. Roosevelt also created wildlife refuges where animals were safe from hunters.