Russia couldn't invade the US now, because it's Blue-Water Navy has been left to rust to junk. They have, what, one or two air-craft carriers, and they are OLD. Russia's new tanks and airplanes are pretty gosh-darn decent - not as good as the M1A2 and the F-22 Raptor or even the F-35 Lightning, but good enough to put up a decent fight.
But even if you have the largest land army in the world, you can't invade the US if you can't cross an Ocean. Russia's Navy is not large enough to transport a force sizeable enough to invade the US. Hell, they don't even have a big enough navy to transport the fuel and ammo needed to sustain an invasion. The only way Russia would ever be able to invade the US, is if they made an alliance with Mexico and spent years transporting troops and fuel and equipment into Mexico in preparation for a US invasion - and if they did that it would be so blindingly obvious that the Russians were planning an Invasion, that the US would do something to stop it. Also, even if they did spend years building up an invasion force, their supply lines would be quickly cut off once war commenced, and the Russian Invasion force would shut down quickly.
China can't invade the US either - and it doesn't want to. Neither does Russia for that instance. They are much more worried about the US invading them or their allies. Russia have invested heavily in anti-ship missiles, which can do some pretty nasty things. The Chinese have converted some of their older Nuclear ICBM missiles into conventional anti-ship missiles - these missiles are designed specifically to be carrier killers - to launch into space and come straight down with a ton of force on a US carrier.
Russia and China know they can't invade the US. If you read military magazines, generals from Russia and China are fully aware that their military technology lags far behind the US and that they are playing catch-up. They have no plans to invade any time soon, and their recent military purchases are all about bolstering their defensive capabilities, not their offensive capabilities. China is very interested in protecting its mineral and gas interests in the South-China Sea and have made improvements to their Navy to reflect that - they want to keep the US out, not invade the US.
But even if you have the largest land army in the world, you can't invade the US if you can't cross an Ocean. Russia's Navy is not large enough to transport a force sizeable enough to invade the US. Hell, they don't even have a big enough navy to transport the fuel and ammo needed to sustain an invasion. The only way Russia would ever be able to invade the US, is if they made an alliance with Mexico and spent years transporting troops and fuel and equipment into Mexico in preparation for a US invasion - and if they did that it would be so blindingly obvious that the Russians were planning an Invasion, that the US would do something to stop it. Also, even if they did spend years building up an invasion force, their supply lines would be quickly cut off once war commenced, and the Russian Invasion force would shut down quickly.
China can't invade the US either - and it doesn't want to. Neither does Russia for that instance. They are much more worried about the US invading them or their allies. Russia have invested heavily in anti-ship missiles, which can do some pretty nasty things. The Chinese have converted some of their older Nuclear ICBM missiles into conventional anti-ship missiles - these missiles are designed specifically to be carrier killers - to launch into space and come straight down with a ton of force on a US carrier.
Russia and China know they can't invade the US. If you read military magazines, generals from Russia and China are fully aware that their military technology lags far behind the US and that they are playing catch-up. They have no plans to invade any time soon, and their recent military purchases are all about bolstering their defensive capabilities, not their offensive capabilities. China is very interested in protecting its mineral and gas interests in the South-China Sea and have made improvements to their Navy to reflect that - they want to keep the US out, not invade the US.