This kind of thing will never happen, I don't know why you Americans have such fear and doubt in your hearts, but I will still analyze it specifically for you.
If the United States is an aggressor, fighting in Asia, and Russia is helping China, then the United States may be defeated. The United States will have to transport a large number of troops across the largest ocean on Earth, which requires a significant amount of mobile power. (Our troops already stationed in Asia are not enough to invade China. Even with sufficient troops, Russia and China may notice that we have withdrawn 30000 soldiers from South Korea and 50000 soldiers from Japan.)
Obviously, any invasion by the United States is carried out after intense disagreements with China and/or Russia, so both countries will closely monitor the movements of the US military and navy. The United States will not suddenly invade.
After this arduous troop mobilization, the United States will have to maintain a logistics chain to provide supplies to troops and aircraft in combat against two powerful opponents with much shorter supply chains.
Then, the United States will have to establish an air advantage to confront the much larger Chinese military, which will have a home advantage. If we assume a full-scale war (everything the United States has over China), then air superiority will be possible - albeit at a very high cost. However, if Russia and China fight side by side, the Russian air force may destroy the remnants of the US air force and establish some form of air superiority. This is where America failed.
Even if the United States passes this, the Chinese military, which has an advantage in quantity, may still win ground wars. Generally speaking, the US military is well trained and well-equipped, but China's numerical advantage, Russia's air superiority, Russian military assistance to China, and the combination of logistics will ultimately erode the US military. Otherwise, China may use its limited nuclear power to eliminate the remaining US military.
If the United States is an aggressor invading Russia, then no one will win, even if China aids Russia. The reason is simple. During the Cold War, the United States and Russia were rivals in global conflicts from Central America to Vietnam and Afghanistan. In the past, these conflicts may have led to full-scale war, but this was not the choice of the two nuclear superpowers. No other country is worth taking the risk of nuclear war and ethnic suicide.
However, both the United States and Russia understand that if the other party attempts to invade the "home country" (the United States or Russia) itself, then nuclear weapons are definitely a choice to defend their homeland. The same goes for today. Faced with the threat of survival, both the United States and Russia would rather risk nuclear destruction of the world than sit back and see failure. For both the United States and Russia, it is' if we are going to collapse, they will collapse with us. '
Russia's population is only half that of the United States, its economic scale is only one tenth that of the United States, and its military budget is only a small part of the United States. Russia's ability to continue a prolonged war will be greatly weakened, while the strong logistical capabilities of the United States will offset any advantages provided by the Chinese military. In the end, Russia will have no choice but to use nuclear weapons to prevent the United States from advancing. This is where war is no longer interesting for the three countries, no one will win.
Many other answers are based on wishful thinking. Well, if you can sink the US Navy, then the ocean will become clean and unobstructed, allowing for land invasion, or if you destroy their satellites, they will become blind and may be defeated, Or "if the entire Marine Corps is too foolish to notice that they are about to enter the edge of Niagara Falls, then... such nonsense. These answers are only valid when the question is" Is there a fantasy scenario where Russia and China may invade the United States?
Of course, you can use submarines to sink our aircraft carriers and use anti satellite weapons to destroy our satellites, but Guess what? The Pentagon is also aware of this, so they are ready to defend against such attacks. The United States has anti-submarine weapon, attack submarines and our own attack satellites. That's why all What would happen if you destroyed their so-and-so... these statements are nonsense. In any war, if you eliminate all the enemies, you can easily win. Indeed, this is the meaning of war. Destroying our satellites, navy, air force, etc. is not that easy, and those who propose these fantasies cannot guarantee that the United States will not cause more damage to the assets of Russia and China in the first place. A military budget of $600 billion annually buys a lot of cool equipment
a) Crossing Alaska/Canada- First, Russians and Chinese must cross the Bering Strait. The United States' air and sea advantages will prevent this from happening. If they did cross the strait, they would have to transport a large number of troops through the frozen wilderness of Alaska and Canada. Although the Russians can better cope with the cold climate, they do not have enough troop transportation to complete this task. China also does not have enough troop transportation and is not accustomed to the Arctic climate.
After marching/driving through the vast expanse of Canada, they arrived in the United States only to find that they had no significant military targets to occupy in the area. This gave the United States enough time to gather defensive forces. Then, the Russian and Chinese armies will face the US Army, the United States Marine Corps, the US Air Force, the National Guard and the whole big country, which happens to have the most fully armed citizens on the earth, and they will have the home court advantage.
b) Through Mexico. Almost all the same obstacles apply, but Russians and Chinese must first cross the largest or second largest ocean on Earth to reach Mexico, which the US air and naval forces can prevent.
The only real difference is that when Russian/Chinese troops reach the US border, they either discover a vast open desert with no important military targets and become live targets, or they discover California or Texas - both with large army and air bases, making it easier to repel invading forces. In Texas, they will also find that 15 million fully armed citizens really enjoy shooting. Admittedly, they cannot effectively defend professional armies, but they will become a powerful obstacle for guerrilla resistance.
Of course, Russia and China must maintain supply lines for all these forces, which can easily be disrupted by US air and/or naval forces. This is assuming that they have the logistical capability to do so, but none of them do.
The advantage of this choice is that there are many large cities worth attacking on both the east and west coasts. However, Russians and Chinese still have to cross the ocean first, and the US air and naval forces can easily prevent this If Russia and China have such logistical capabilities, they do not. Moreover, once again, a successful crossing only means that they will face the concentration of major land and air forces in the United States - some of which are located On the ocean coast.
To be honest, this is a bit like the other side of the fantasy. Russia and China together hardly have enough air logistics capabilities to transport the large number of troops and equipment needed to invade the mainland of the United States. Even if they do, they must first pass through the US Air Force, which is not easy. After doing so, they will face all the other obstacles mentioned in the aforementioned land invasion.
In addition to all the obstacles I have already mentioned, the United States has other defense measures that I did not mention because this article is already long enough. Russia and China are almost impossible to obtain enough troops and equipment to invade the United States, and even if they can, they are almost impossible to win the war.
Finally, let's take a look at the other side of the Russian scenario. If Russia and China really manage to reach the United States and successfully invade it, then there is nothing that can prevent the United States from using nuclear weapons against invading forces or against two invading countries. In this highly unlikely scenario, the United States will be 'defeated', but victory is not 'worth it' for either Russia or China.
Similarly, it all depends on what kind of war we are talking about. If the United States were an aggressor in the Asian War, then we would be defeated. If the United States is defending its homeland, then it is almost impossible for anyone to defeat us.
In military terms, any attack on the United States will be an attack on all NATO member countries, and the United States' joint defense agreements with Japan, South Korea, Australia, and New Zealand also have similar provisions. Therefore, this will not only be the confrontation between Russia and China (and possibly North Korea) and the United States, but also between Russia and China and NATO, Japan, South Korea, the Philippines, Australia, New Zealand and monarchy countries in the Persian Gulf (such as Saudi Arabia). In such a large-scale war, European non NATO countries that do not border Russia, such as Sweden and Switzerland, are likely to join NATO. Israel will provide assistance as much as possible.
India is an interesting exception - millions of Europeans, Americans, Canadians, and Australians are of Indian descent, while there are very few Indians in Russia and China. India has never been a friend of China. So, India will not be on that side. India is most likely to remain neutral, just like in the Cold War, which means Pakistan will remain neutral (not letting India expose itself). Vietnam and Iran will not participate, but the reasons are different. None of the five Stan countries will participate. Georgia, Azerbaijan, and Armenia are also involved. Most countries in South America and Africa will stay out of it, why bother themselves?
Now, with some new, unforeseeable, and asymmetric weapon systems or methods, let's consider the economic aspects of this issue. Let's take a look at China's main trading partners:
Therefore, any attempt by Russia and China to "defeat" the United States will immediately push them towards a larger and more advantageous group of hostile countries. Especially, China will lose oil supply from the Persian Gulf and iron ore supply from Australia. Both Russia and China will lose a large amount of technology and materials they need to launch a war. In addition, any progress in "defeating" the United States would undermine China and Russia's foreign trade, which they heavily rely on (the United States is more self-sufficient than most countries, outsourcing manufacturing for cost reasons rather than a lack of technology).