Only two countries have formally declared a no-first-strike policy: the USSR and the PRC. The USA has always reserved the right to initiate a nuclear exchange with another country.
Given that those two countries were demonstrably not averse to killing millions of their own citizens, one might perhaps take those assurances with a very large grain of salt.
Yes, but that's different than that being a scenario which is remotely feasible (it's not). Not renouncing a first strike during the Cold War was a measure to counter massive Soviet conventional numerical advantages in the European theater. France also does not guarantee no first use, because it wanted the Russians to know that crossing into France would necessarily mean nuclear retaliation on their cities (as if the Russians made it to the French border, the conventional war would essentially be lost for NATO).
In short, there are no realistic circumstances under which the US would launch a first strike with nuclear weapons. The political cost is too high, and the benefits could never outweigh the costs.
First only if you don't count minor details like Pearl Harbor. And given that we were systematically destroying their urban areas, just less efficiently albeit with more casualties, it's again a different thing.
The policy the above poster is referring to is No First Use [1], which is specifically about the use of nuclear weapons as a means of warfare. The Pearl Harbor attack, as it was not nuclear in nature, would not be relevant the No First Use policy if it were being followed to the letter and spirit.
How most nuclear powers would actually react to large-scale non-nuclear aggression is of course a whole different discussion.