The Russians are smarter than us; they slowly wear you down, slowly destroy your will to fight and economy. Theyve never been conquered, and they had to deal with HItler, Napoleon, The Romans, The Swedes, The Turks, The British.
Since WW2, the USA has only made war with third world countries and have mostly failed to achieve victory; Korea, Vietnam, most recently to Iran. American tax payers spent 4 trillion dollars invading Afganistan. Who is still in charge? The Taliban.
China has factories literally hundreds of feet below ground, where the B2 spirit bombers cant damage. Same with Iran's missile making production facilities. America at war is not about conquering, its about making weapons, sold by politicians who are really just gun salesmen, bribed to terrorize tax payers into funding wars that are never won. Desert Storm One. America had 900k troops and a national debt of only 4 trillion. The debt is now 40 trillion and America can only afford to pay half that number of soldiers.
China meanwhile, is slowly replacing America as the world leader. If China and USA fought, one on one, China would win. Its logistics. China's production capacity is way bigger. They make most of the worlds cars, in factories that can pivot and build tanks, jeeps and humvees, in a few days notice. Bigger population, more soldiers and communist countries get war weary, long long after Democracies do, since their citizens do as their told.