to be honest i dont think any of them are a threat.
Syria= They have nothing that can hit america
Russia= They have downgraded their entire military, and they are trying to become allies with the US, so why would they want to reck that?
North Korea= Same as syria, but the US is most likely to hit NK, so in that case the Koreans would pose a threat
China= Large Military, but lack long range equiptment.
Iran= outdated equiptment and lack of funding. i suppose you could say that they owe america for taking out Saddam
Mexico= They would have the surprise, but being so close to america, logistics would be no problem for the US.