So. Hmm. Where to begin.
The ~200 countries in the world are not all the same size, and neither are their economies. If Tuvalu, say, started burning unicorn carcasses in open pits to heat water to drive wind turbines to generate electricity ... well ... that'd have no measurable effect on global warming. In fact, if the bottom 50 or 60 economies ALL started burning unicorn carcasses in open pits etc etc, it'd still make bugger all difference. Those countries can and should do something to reduce their emissions (if for no other reason than all the bad karma that comes from burning unicorns), but in terms of continuing, halting or reversing global warming anything they do is all but irrelevant.
The US economy is the largest single economy in the world. And not just by a little bit. It's twice as big as China, the next biggest. The US is as big as No.s 1, 2, and 3 combined. The US is over 20% of the golbal economy. The US is larger than the bottom 194 economies. The bottom ONE HUNDRED AND NINETY FOUR. Out of 211 in total.
So let me rephrase your statement, to make it more accurate:
If the US passed sweeping carbon emission laws tomorrow in an effort to combat climate change it would MAKE A HUGE DIFFERENCE.
Pretending 'the US is just one lil ole cog in a huge machine, and shucks nothing we do could make any difference' is only slightly less abusrd than pretending the climate change isn't happening at all.