Belgium, Bulgaria, Canada, Czech Republic, Denmark, Estonia, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Turkey, United Kingdom, and United States.
The above are the 26 members of NATO and bound by Treaty to protect one another. Even Russia knows better than to cross NATO, so no, Russia will not attack Poland for any reason.
The US will have no option but to abide by the treaty but there will be so many countries in the mix that the US would probably have to do very little. Most of the members of NATO are Russia's neigbors and can pressure Russia far better than the US.