After nearly a decade in Afghanistan and Iraq the U.S. Marine Corps wants to get away from being a U.S. Army auxiliary and back to being an amphibious strike force. Talk in Congress about "what do we need two ground combat forces for?" adds to the urgency. The marines have always been sensitive about criticism that they are a second army, a second ground combat force that simply duplicates what the U.S. Army does. In terms of active duty forces the marines are about 40 percent the size of the army. Add in organized reserves and the marines are closer to 30 percent the size of the army.
The marines can perform the same jobs as the army but consider themselves mainly an amphibious force trained for assaults and other difficult special operations. These are things the army has also done but the marines invented modern amphibious operations during the 1930s and 40s and continue to specialize in it. The marines noted how the British Royal Marines went on to help develop the modern concept of commandos and went in that direction as well.
With all this in mind, in the last decade, the navy and marines have sought to reorganize into 12 ESGs (Expeditionary Strike Groups; i.e., a reinforced battalion of marines and their amphibious ships, including a smaller amphibious aircraft carrier). While not a new concept, the ESG was meant to add some new twists to an old idea. This has been difficult because of the need to send marines to Iraq and Afghanistan. September 11, 2001 came as a surprise to the marines as well and they had to adapt. The marines like to consider themselves good at adapting.