将下面这篇文章的第一段重新排列，使其更加准确地概括文章的主题。
Original paragraph: 
"Theodore Roosevelt became the 26th President of the United States after the assassination of President William McKinley in 1901. At 42 years old, he became the youngest President in the nation’s history. Roosevelt was a leader of the progressive movement, which sought to address various social and economic issues in the United States during the late 19th and early 20th centuries."
Roosevelt, the youngest President in US history, was a progressive leader who aimed to address social and economic issues during the late 19th and early 20th centuries after becoming the 26th President of the United States in 1901 due to the assassination of William McKinley.