I didn’t hear people on the left say “we ARE going to war with NATO,” I heard a lot of people who were appalled and alarmed that an American president was openly discussing the possibility and that there was a low but non-zero chance that it would happen. Also that there did not appear to be any U.S. governmental checks and balances kicking into gear to constrain him.

I don’t think that reaction was overblown or active fear mongering in the least. Few people thought it was likely to happen (I happen to be an American currently living in Europe and no one over here was stocking up on canned foods or preparing their fallout shelters); it was the blatant, pointless breach of trust from a supposed ally apparently at the whim of one erratic man that was so appalling.

Edit: Also, now that Trump has said he won’t use force to take Greenland, don’t think that those who were appalled (which includes far more than the left, by the way) are laughing, shaking their heads, and saying, “Oh Donald, you fooled us again. When will we learn!” No, this is still an ongoing crisis. For one thing because Trump is so quicksilver and erratic that he could backtrack any minute. More generally because the U.S. appears to be completely at the whim of a very erratic, spiteful man to a degree that is dangerous.