Why? We launch nukes against your nuclear forces, destroy most of them and then suggest peace. You may agree, and then less than one million of Americans will die from fallouts, mostly in Wyoming and Montana (and who cares about Montana?) or you may disagree and retaliate. Your retaliation strike will be crippled and poorly coordinated, and will be weakened by Russian ABD, and it effescts will be elavuated by Civil Defense and Emercom. So, it won't kill more than one million of Russians, but after that the third Russian strike will destroy US infrastructure with direct kill over 30% of American population and 90% losses in one year (mostly by refugees).And should you lauch Nukes you will die young along with most of the world.
And one million of Russians and 100 millions of Americans are definitely not "most of the world".