I don't think US has enough weapons and technology to conquer the entire world. That's indeed a stupid statement.
Having an Indian background, as far as I know, there is no known counter weapon in the entire world (including US), that could stop a BrahMos - a joint missile project by Russia/India.
Weapons and technology is one thing. There is another called 'strategy'.
Iran 'strategy' : When war ships gather and provoked Iran in Oct 2007, it issued a statement that 11,000 missiles would be fired within the first minute of attack. Now tell me how many aircraft carries and war ships can stop 11,000 missiles in 1 minute? Don't think that when Iran sends a missile, it is going to come alone which the US can spot and hit back with another missile. Iran will send 1000s of missiles in a shot time to one single ship. Even if the ship is so advanced and costs 12 b$, the survival rate of the ship is next to 0.
Chinese 'strategy' : Chinese strategy in the world arena is very silent, clever and sucking the wealth and power of US slowly like a leech undetected. If US goes to war with China, I can bet you, US is destroyed for ever. The reason being, US is not financially sound at the moment for a war with China. If US has to go for war, China needs to lend US 40% of the cost for it's war by buying US bonds and finance the US which I think will never happen against China itself. If such a threat is found, China will dump 1 trillion $ into the market which will devalue dollar and trigger hyper inflation in US. This will make US so weak financially that it cannot even fund a single day of war. Apart from a financial strategy, China is no way inferior than US in military.
Russian 'strategy' : Russia has twice the number of nuclear weapons than US. Fighting against Russia is a suicide for US. For every city US is going to fire a nuclear weapon, Russia will fire against 2 cities in US. Russia has 12,000 nuclear weapons while US has only 5,113 nuclear weapons.