Who served as president of the united states from 1901 to 1909

 Theodore Roosevelt (often known as "Teddy Roosevelt")

Further detail:

Theodore Roosevelt was vice-president for President William McKinley, beginning that service in March, 1901.  When President McKinley was assassinated in September, 1901, Roosevelt became president.   He was the youngest ever to serve as President of the United States at the time he started in the office -- at age 42.

As president, Theodore Roosevelt saw the role of government leadership as "steward" to the nation, working on behalf of everyone's interests, not just the interests of any powerful small group.  His "Square Deal" policies were aimed at making things fair and square for the general public.   He is also known for the vast amount of land  set aside under his administration as national forests, national parks, national monuments, etc. He was seeking to protect the use of the land for all Americans' interests, rather than letting corporations tear into any land or forest they wanted in order to grab natural resources.