The US 3rd Division had the most time in combat during WWII,

517 days in combat of the 34th Division, during WWII is the record for any Combat Division for the US Army (Elements of the 34th division were active in 611 days of combat).

https://en.wikipedia.org/wiki/34th_Infantry_Division_(United_States)#World_War_II

Remember, WWII for the US was from December 7 1941 to September 1 1945, i.e. three years (1095 days), ten months (300 days), and 24 days or 1422 days (January, March, May, July and August had 31 days, thus an additional five, days, but subject two due to February only has 28 days). Thus the 34th Division was in combat less then 44% of that time period (Most Combat units were in Combat even less time, the 34th had the MOST days in Combat).

You hear the reports of heavy combat in the islands and Anzio and Normandy (and the various "lines" the Germans made in Italy) but once the line was broken, or the allies broke out of Normandy, heavy combat dropped as the armies raced to the next location where they would fight. There were combat in these "Flights" but much more limited then where the Germans or Japanese took a stand.

In actual time in combat at one time, the 3rd infantry division still holds that record of over 100 days in combat at one time (Most units had "Breaks" where they were withdrawn from combat. Studies indicated that if a unit lost 10% of its personal, its effectiveness declined rapid. This decline is due to you going from a 15,000 man "Team" who have learned to work as one "team" to 15,000 individuals who barely knew each other forced to work together. You can compared it to the Pro bowl team vs the Super Bowl team. Technicality the Pro Bowl team is better, but it had less then two weeks to become a "Team" while the Super bowl team has been working as a team for over five months by that time of the pro bowl).

The breaks in combat are designed to get the unit back working as a team. Thus most units had "Breaks" to fill in losses and get people working together. This is by far the better way to integrate new personnel into a unit, then sending in replacements to units in the line. Replacement on the line was adopted during WWII by the US Army due to limitation of transport. It took less efforts to move people then units and transport was the main restrictions as to US military units in WWII. The Army did a study that it could raise 267 Divisions from the US without affecting the ability of the country to supply the Military. The problem was the Navy pointed out they could only transport and supply 100 divisions. The US Army fielded 90 Divisions (and the Marines 10 Divisions).

Given this restriction replacement on the line appeared on its face to be a way to keep those divisions up to strength. The problem was you ended up with a division full of individuals not a team, thus over time in combat the unit effectiveness would drop. The US Army quickly saw it was NOT working but was so committed to it that they continued using replacement on line till after Vietnam (Where the system was finally shown even to the bean counters as NOT to be workable). Thus units were moved into Iraq during both Desert Storm and the 2002 War to "liberate" Iraq (and the war in Afghanistan). Fortuitous, the US Army did not need the size Army it had sent into Vietnam so the US Army could move units in and out.

I bring this up, for even in Vietnam enlisted personal only saw combat for 365 days, then had to be rotated out. Officer only saw 180 days in front line duties, then transferred to headquarters to get some experience at that level. Many personal did make various "Tours" of Vietnam, but none for more then 365 days at a time. Please note this was the official "Policy" there were exceptions to these rules, thus some people served less or more time depending on what was going on at that time period.

On top of this most wars involved getting the armies to where the battle will take place. Today that is a lot easier then in the days of olde. i.e. we can fly troops to the battlefield instead of having then March to the Battlefield (and most Armies moved no more then 10 miles a day when they had to travel on foot, the Red Chinese Army into Korea in 1950 was one of the fastest movement of troops up till that time, averaging over 20 miles a day, Rommel of North Africa Fame only averaged 13 miles a day).

Side note: Since Vietnam, you have seen a rapid increase in the speed of land warfare. Today it is possible to move a unit 100 miles a day, which includes moving not only the troops and their vehicles but all of the support elements, fuel, food, repair stations etc.

The Average speed on the INTIAL attack on Baghdad was only six miles per hour, fast for a traditional army, i.e. 60 miles in a ten hour day:

http://www.bu.edu/globalbeat/syndicate/corbin042103.html

The Actual ground offensive lasted only nine days:

https://en.wikipedia.org/wiki/Battle_of_Baghdad_(2003)

The distance from Barsa, on the Persian Gulf to Baghdad is 340 miles, thus it took nine days to go from Barsa to Baghdad or 38 miles a day.

Now, the US Army stayed in the surrounding Desert, going twice or three times as long as the direct distance between those two cities, but that comes to no more then 100 miles a day average. It is hard to move an entire city (and that is what a Division is) 100 miles a day.

To do this every US Division of about 15,000 men, used the same amount of fuel as an WWII era US Army Corp, of about 100,000 men. I have NOT read of how much the corp use increased but a factor of ten would not be unreasonable. Each Modern Division will be using as much as a WWII Army corp, thus three WWII Army Corp usage. Each Division tends to be supported by Corp elements of about the same size, so another three WWII corp equivalent, plus the additional fuel for Corp only elements like Attack Helicopters AND increase fuel needs to haul the increase fuel needs of the other elements of the Division. Thus a Modern Army Corp uses about 10 times what a WWII Army Corp is not unreasonable. Yes you have increased speed but sooner or later that tail has to catch up and as such the tail slows you down, but without it you have no fuel to go faster then one can walk.

Sorry, the cost to operate a modern offensive army is high, and maybe to high given where will the US use its Military in the offensive to defend the US? So the US has the capability to be in Mexico City in Two Weeks, why would the US want to have its Army in Mexico City? The same in most other parts of the world. Why would the US want its forces on the Yalu River, the NORTHERN border of North Korea? Another two week project of questionable worth. What about reaching Moscow from Estonia and Latvia? The US Army is capable of such a maneuver once it builds up forces in either of those two nations, but once the US has Moscow, the what? The Russians will do what Iraq did, go into guerrilla war and that war would make the Iraq look like a picnic (I down play nuclear weapons for even for Russia, a Guerrilla war is the better option).

Side Note: Once the US has Moscow, the Russians will still have access to their ICBMs, so that would the US have gained beside a stalemate?

Shock and Awe works great on a fairly sophisticated country, but against countries that are mere bureaucracy over Tribal people (Most of Africa for Example) Shock and Awe will Shock and Awe only the people who really will NOT resist the US Invasion. To a degree that is true of most of Latin America and Asia. We will end up winning the first two weeks of the war, and then losing all we have gained AND MORE, within ten years due to guerrilla activity.

Sorry, it is time to cut the military big time. We are no longer facing a Communist Second war who was committing to Conquering the West and spreading communism every where. Do we need 12 carries and more Fighters then then the rest of the world combined? Do we need an army 2/3rds of the size of the Army we had built up to defeat the Soviet Red Army during the Cold War?

In short the answer is NO, we do NOT need this large military force to defend the US. We do need it to protect corporate interests overseas, but is that a US National Security interest? I do not believe so, US interests ends at the water edge (or the 200 mile economic border) not all over the world.