Russia and the USA can fight for decades and decades but we both know that avoiding nuclear war is the number one concern. It will lead to MAD (Mutually Assured Destruction) basically wiping everyone out and returning to the stone age. We would fight just to the brink of MAD and then both Russia and the US would back off. Both countries are smart and wont use nukes. Lets say Russia invades the US. The US would win. Why? Because of the right to bear arms. We have a massive militia. If they want to invade they have to go door by door because the average American has 1 gun per household (this is an average meaning not all house holds will have a weapon). Now say we (the US) invade Russia. Russia is HUGE. Very strong military and a lot of land to cover. Chances are we would lose. It all depends on where the attack is taking place, are there other countries getting involved? If so it will probably cause another chain reaction (Like the Islam/Iran theory) and lead to another World War. We know this. Thats why we wont make the first move.