The United States in 1903 was clearly an imperial power. It is possible to argue that the US was a relatively benign imperial power, but it was an imperial power nonetheless.
Imperialism can be defined as a system in which one state dominates one or more other states. The dominant state would be seen as an imperial power. The United States, in 1903, was clearly an imperial power by this definition.
In 1903, the US was trying to dominate countries as nearby as the Caribbean and as far away as Asia. The US had just finished a war to subjugate the Filipinos. It was pushing for access to China that was equal to the access that other imperial powers like Britain and Japan had. In the Caribbean, it controlled Cuba and was helping Panama to break away from Colombia because it wanted to dominate Panama and build the Panama Canal.
All of this clearly shows that the US was trying to dominate other countries. It felt that it was racially and culturally superior to them. It wanted to “improve” and “civilize” them. It also wanted more economic and military power. All of these factors combined to push the US to act as an imperial power in the early 1900s.