Treat the earth well: it was not given to you by your parents, it was loaned to you by your children. We do not inherit the Earth from our Ancestors, we borrow it from our Children. ~ Ancient American Indian Proverb
Iran has a population almost 3 times that of Iraq and their military hasn't been debilitated by a war and 10+ years of sanctions
Iran is correct in their assessment that, we don't have the manpower or the national willpower to fight a long, protracted war against Iran.
China stated a long time ago that they were planning on a war with the US.
Russia and China already sell weapons to Iran and give them technology. Russia and China will support Iran because it hurts us and anything that reduces our strength, economically and militarily, and our influence, creates a vacuum that they can move in on
China is using a lot more oil then they used to. Making friends with a major oil supplier is a good move for them, especially when it cramps our influence in a major strategic area and right now they are sucking in huge bucks selling us stuff and making our economy dependent on their industry.
Historically speaking, America has had its shot. China knows this and are well aware of the grave internal issues America faces.