Did American involvement in China in the early 1900s constitute informal empire-building? If so, how?

American involvement in China in the early twentieth century constituted "informal" empire-building in much the same way American involvement in Cuba and the Philippines did. In both cases, the US deployed troops in order to protect American economic interests.
While never creating a “formal empire” in the same sense other nations (like Great Britain, France, and Russia) did during the early twentieth century, the US did control areas and nations across the globe. Many influential leaders—men like Theodore Roosevelt, Henry Cabot Lodge, and William Randolph Hearst—believed that the US needed to expand across the globe in order to increase its global influence and to strengthen its economy, all the while showing other nations its military prowess.
The US began expanding globally following its victory in the Spanish-American War (1898) by gaining control of Cuba, the Philippines, Guam, Samoa, and Puerto Rico. During this period, the US also annexed Hawaii. By controlling these areas, the US could exert its influence economically and militarily. The same thing occurred in China in 1900. Various nations had created “spheres of influence” in China; these nations controlled the trade in these areas. Basically, these nations wanted to keep trade flowing between China and the countries that controlled the various “spheres.” The US was one of many nations with a “sphere of influence” in China. When Chinese nationals tried to expel foreigners from the country in what would become the Boxer Rebellion, all countries that had an economic interest in China allied together (militarily) to crush the rebellion. The US, along with other nations, continued to manipulate and control areas of China.
The US controlled areas of China for its own economic interest and maintained that control through the use of military force. While lacking the formality of many of its contemporaries' “empires,” the US empire no doubt existed, albeit with less conventionalism.