The most obvious example of US willingness to be sufficiently brutal seems like Vietnam
Um, no. The US was actually extremely soft in Vietnam. However, photographs from the war were presented in a misleading manner to domestic US audiences to make the US seem brutal (e.g., the infamous Vietnam execution photo was a guy killing a Viet Cong assassin who had just killed his family). Thus, the US lost its will for even that level of brutality and promptly lost a war it had been tactically winning.
Um, no. The US was actually extremely soft in Vietnam.
Um, no. The US actually dropped "one million tons of ordnance" on North Vietnam (where US air strikes killed "approximately 52,000 civilians") and "some four million tons of bombs" on South Vietnam ("the most bombed country in the history of aerial warfare—a dubious distinction for an ally"). The US actually supplemented those bombs by dropping about 70 million litres of herbicides on Vietnam, including over 40 million litres of Agent Orange and Agent Orange II...
Here's my op-ed that uses long-term orientation, probabilistic thinking, numeracy, consider the alternative, reaching our actual goals, avoiding intuitive emotional reactions and attention bias, and other rationality techniques to suggest more rational responses to the Paris attacks and the ISIS threat. It's published in the Sunday edition of The Plain Dealer, a major newspaper (16th in the US). This is part of my broader project, Intentional Insights, of conveying rational thinking, including about politics, to a broad audience to raise the sanity waterline.