If the US and Russia tried, they [i]might[/i] be able to, but it would be difficult to get over the sheer numbers that China and India have. Plus, China's one child policy has led to a huge boost of boys that are starting to approach the military age. Even with all of our tech, it wouldn't be easy to defeat what could easily be 1-1.5 billion troops, both professional troops and militia, that could emerge from India and China alone. Also think about how much the US imports, just from China alone. If we declared war on China, we would be in some serious trouble, financially. I'm not as familiar with Russia, but I don't think they are the military power they once were. During and soon after the world wars, they had the largest army in Europe, simply because they are massive (they were still agricultural, which meant families had a lot of kids to work in the fields, lots of kids means lots of soldiers), and being the second country with nukes secured their spot at the top, but now that there are so many nations with nukes, and your rules state no nukes, there isn't anything particularly special about Russia. Much of the country was hastily assembled during their rapid industrialization, and is crumbling. I also don't think Russia has a great sense of national pride like it used to, so it might not be as easy to motivate troops.