Background
Though the 19th Century the US Marines were largely a ships guard. Their role in littoral warfare limited to raids, & occasionally reinforcing larger landing forces. For the large landing or invasion operations, such as the ASW or the Spanish American War the US Army provided the regiments & corps for extended and large scale ops. After 1898 the growing overseas commitments led the USN to examine the problem of defending temporary & permanent overseas naval bases. The Coast Artillery were a logical source for this mission, but outside Panama there was little interest in the War Department for providing defense units for overseas naval base defense. This led the Dept of the Navy to authorize the formation of base defense units, using the Marines & Navy personnel. As the demand for intervention in Latin America increased the Army was not tapped for expeditionary forces. For events like the Hatian occupation the Navy was tasked & the Marines were expanded and formed into larger permanent regiments. By 1914 the Marines were well on the way to becoming a standing land army owned by the USN. Continued operations in the Caribbean in the 1920s ("Making Latin America Safe for United Fruit".), and detailed development of War Plan Orange led to the Navy expanding the Marines, forming permanent combined arms 'Expeditionary Brigades'. The Coolidge & Hoover administrations continued and expanded the use of the Marines as occupation/police forces overseas. A combined arms force of a rifle regiment and aircraft group were deployed to Nicaragua. A regiment stood up in China. A Expeditionary Brigade was sent to Shanghai in 1927 & returned to the US in 1928.
The US Army had a overseas role in this era limited to permanent forces in a few areas. Panama, Hawaii, & the Philippines. The only significant temporary intervention being the move of the 15th Inf to China from 1927 to 1938.
The Navy was also stuck with the task of developing amphibious warfare doctrine largely on its own. While there was a Joint Board established early on the Army side concerned itself mostly with Strategic questions & planning. The tactical & technical side were left to the Navy & its Marines. This extended to the planning for the Orange plan. The Navy & Marines studying the problems in detail, while the Army largely confined itself to the question of mobilizing a 100,000 man force for use overseas.
So WI?
The PoD being very early 20th Century, with the War Dept directing the Army to provide expeditionary units for the Navy. Base defense units, occupation/police forces in Latin America. The Marines are not much expanded post 1898 & remain a collection of individual rifle companies & temporary battalions. How does this affect things post 1914, the Great War, Latin America, WWII, ect...