lol, it sounds like reading a book about Roman Empire
The US is not the Roman Empire. The Roman Empire when it was beaten retreated The US has continued.
After Vietnam, The US continued, After Korea the Us grew stronger.
Rome Only extended as far as the Med and Europe, She Fell when She extended her self beyond her logistical capabilities, Shrank her legions and then sub divided herself into two Empires The Failing Western empire (Rome) and the wealthy eastern Empire ( Constantinople, Modern day Istanbul) the critical fail for Rome that lead to it's sacking was a betrayal when the Romans Offered lands to the the displaced Gauls and then reneged. Before the fall of Rome, she fell into economic decline. moving to the second largest economy of the region the first being Constantinople.
Constantinople lasted until the collapse of the Crusaders, when corruption and Scheming backfired.
The US is placed in a position where she needs a strong Navy and Air force, as The US is both as much a European state as England as much a Asian state as Japan. She is located in the critical point bridging the two oceans of the Atlantic and the Pacific. Despite faltering she remains the worlds first economy.
now back to it. why the 400+ Ship navy. First lets be honest here the number given is not set in stone. the number has been fluffed. Congress likes to make it's own mind and usually they cut down the numbers. it's a negotiation, Haggling between the DOD and Congress. Sometimes it works and the needs of the Service are filled some times like with the Raptor the political wins and the needs of the service are neglected. I would say the number wanted is likely closer to 400. still larger then Europe. Why so big? First LOCATION LOCATION LOCATION! European nations in the defensive mode are vulnerable from three Seas each of these seas has a number of Carrier task forces the North Sea ( UK pending) the Atlantic (UK pending, France, Spain) and the Med ( Spain, Italy).
The US? east Coast. The North atlantic, The Gulf, the South Atlantic.
West Cost. the North Pacific, The South Pacific, Indian Ocean/ Sea of Japan ( Guam and Japan due to strategic and Treaty)
the Pacific is huge territory far bigger then the Atlantic. demanding more assets and resources then the Atlantic ever could The next biggest player in the America's is Canada, but Canada has a much smaller naval fleet then the US and a much smaller area of open Coastal vulnerability. Canada and the US have a long Running relationship. the two nations the United states and Canada are sometimes accused of being one and the same. moving south is Mexico. Of the Continent of North America there are three full nations Canada, the US and Mexico. Of the three the weakest player is Mexico. So the Us bares the brunt of Defense for the three nations.
Next Nato and Europe have been until very recently the Junior partners in global operations. The Us has employed every where Nato has, Nato has not Employed everywhere the US has. The USN is one of the busiest multi-mission services on the planet. the Others the USAF the USMC and the US ARMY. The US is everywhere, and not just fighting. They are building schools, supplying medicines, treating the Sick. The Us has defense commitments far beyond it's own territory, So having a fleet far larger then NATO and Europe is reasonable. Additionally European navy's have a strange habit, They transition from Naval power to coast guard on a regular basis. the French Navy employs it's CVN, Then pulls her back into port for refit. they loose there carrier. the UK right now just retired all it's harriers and it's true carrier force. The USN would never do that. by keeping a large carrier force. the US can Cycle it's Carriers in for refit and repair well maintaining the same active strength.
Sadly the USN has been sapped over the years. the Last American president who saw it as a asset was Ronald Reagan. President Obama is just the latest of the run of Presidents who have viewed the worth of the USN as lesser then the needs either pressing or presumed of asymmetric conflict. it's not just the USN though the USAF lost B2 and F22's the US Army was pushed to cut Armor and Artillery the USMC has slashed out it's Amphibious capabilities in favor of lighter crafts and Mraps and brown water navy, keeping only the most basic of deep blue water assets.
The newest ships in the USN are the Ford Class, With the Ford now going through Trials, The LCS freedom and independence, Virginia Class SSN, the Zumwalt class Destroyers, America Class LHA, Spearhead Class JHSV, Montford Point-class Mobile Landing Platform, Lewis and Clark class resupply ships, San Antonio Class LPH, and the Armstrong-class oceanographic research ships and the M80 Stiletto Special operations Ship ( what I would call a modernized PT boat)
Now Zumwalt production has been cut to three, and both Independence and Freedom LCS are both being cut in favor of Frigates that likely would be variants of the same to replace the Oliver Hazard class which is well beyond it's capabilities.
The termination of the Zumwalt is short term to restart the Arleigh Burke class but the Arleigh's are more Littoral in nature and some are projecting the Burkes until the 2040's. the cancellation of the CGX in 2010 places strain on the fleet as Destroyers will have to take the place of Ticonderoga-class cruisers that may be retired early. a short term suppliant the needs is a Ballistic missile defense ship based on a existing hull like the San Antonio class but that is basically a arsenal ship minus all the sexiness and multi mission capabilities . The Ford Class is needed to replace Nimitz class Carriers now reaching there 50 year end of life. Lewis and Clark are self explanatory, Armstrong is scientific.
America class is needed as the existing class is reaching it's operational end of life soon and as Amphibious fit along with the San Antonio, and Spearhead in Littoral battle sets with Stiletto sitting nicely.
Well all of this seems fine and good against a lesser state with limited to nonexistent naval assets, the World has changed. The PRC has built a capable naval force and continues to improve it making a possible peer strength, they have also been pushing there wants in territorial disputes placing American treaty allies in a position of feeling threatened. they have a technologically sophisticated force and extensive ship building capabilities.
The Russian Fleet once a rusting hulk has come a long way, They remain one of the worlds top military technology exporters. there recent sale to India and procurement of Mistral class LHD with the intention of indigenous production shows a nation moving back to carrier production capabilities. and a nation willing to export said carriers. they sell SSK and SSP type ships and are quite happy to do so.
North Korea, Iran and Cuba have been trading technologies particularly in North Korean SSK class submarines, The Iranians have expanded there fleet now to include a number of emerging Destroyer types Although based heavily on hulls they received before the revolution form the US and UK, they al have to start somewhere. the Iranians also have access to more up to date Russian SSK types and the Cuban's although having a much less powerful naval force are know to be in the act to having a number of Yugo based mini subs and recently introducing two frigate class ships of there own design.
The world is a busy place
With the Instability in Venezuela and Argentina, Peru, Columbia, a unpredictable Cuban nation and Mexico in a seemingly undeclared civil war that stretches from the Jungles and Plantations of South America across central America, North into the American midwest.
In Africa Libyia, Liberia, Mali, the CAR, Somalia, Somaliland, Uganda, The Demicratic ( it's not) Republic of Congo( also not), South Sudan, Sudan, The Gambia, Senegal, Nigeria, Algeria, Tunisia, Niger... I mean other then South Africa pretty much the whole Continent is a giant war zone.
The middle East, Israeli Palestinian conflict, internal conflicts in Iran, the Iraqi-Syrian-Lebanon Conflict, Insurgency in Yemen, Egypt. Iran-Iraq-Syria-Turkey Kurdistan issue
Rising troubles in Eastern Europe the Ukraine, Insurgency in the North Caucasus of Russia, and possible issues in the future.
The long term issues between Pakistan and India, Internal issues in both as Maoist fight in India and the Taliban in Pakistan, Afghanistan, The Xinjiang conflict in China, Ethnic Conflict in Burma, South Thailand insurgency and the Korean Stand off.
Off shore you have the Philippines with insurgencies, the Papua conflict in Indonesia, the long standing three way Sino-Korean-Japanese tensions, the Taiwan issue, the long standing territorial disputes.
not to mention Earthquakes, Tsunamis, Viral outbreak, Hurricane, Typhoon, land slide, random acts of terrorism, Search and Rescue, Famine, Drought, wild fire, Flooding.
Look over this list and look hard you will find the USN nearby.