US Military Theatres of Command (Source)
Even Russia and China do not have such a large global span.
Russian Military Districts (Source)
Chinese Military Commands as of 2016 (Source)
When a military is divided across the whole world, bases and military projection capability become very important. Each theatre of command includes further subdivisons of army, navy, air force, marines, forces in base camps and special forces. The US operates over 800 bases outside of US territory. The French, British and Russians operate under 30 each. Land bases are however stationary and can be at risk of being destroyed, so the better alternative would be to have a mobile base with both strong defensive and offensive capabilities. For that, you don’t get more sophisticated than carrier task forces. Take the US Pacific Fleet for example.
Just how big is the Pacific Fleet? The Pacific Fleet controls two fleets which each control in five aircraft carrier task forces in total, as well as submarine forces and a whole host of other auxillary and support vessels. As a simple comparison, the US has 10 aircraft carriers in service, the nearest country is Italy with 2. China and Russia have one each. And even then, these other carriers do not really match up to the US Nimitz Class carriers.
The Nimitz Class carrier is so advanced, older carriers like the Midway Class are used as museums.
The US’ powerful navy needs however home bases on the US mainland and for the Pacific Fleet, that home port is San Diego.
US Naval Ships in the San Diego Naval Base in the background. USS Midway Museum in the foreground
The San Diego Naval Base is considered the home port of the Pacific Fleet, the port that is on US mainland. It is one of the largest bases in the United States and has some 20,000 soldiers and 6,000 civilians on its base. Including a boot camp for the training of recruits.
San Diego is a major military base within the US Pacific Command (USPACOM) and serves as a secondary base for the US Pacific Fleet (the main base is at Pearl Harbour, yes, that Pearl Harbour). It houses some 54 ships
The city’s role as a naval base is not recent. Within the boundaries of the city are at least two naval bases (San Diego and Point Loma), it had been a base since the 1922 and was once home to a thriving defence industry.
With the US expected to build up its firepower even more over the coming 4 year presidential term, one would imagine that the role of San Diego in the defence of the West Coast will probably continue and perhaps even grow.