Answer: "The supreme necessity" is a phrase used to indicate that something is of the utmost importance or necessity. In this context, the phrase "the supreme necessity of avoiding a two-front war" suggests that avoiding a war on two fronts was an issue of the highest importance, and was considered a top priority by the American officials involved in the talks.
The use of the word "supreme" emphasizes the significance and urgency of the issue, and suggests that it was a matter of utmost importance that needed to be addressed immediately. The phrase implies that this issue was not just a priority, but rather an urgent and critical matter that required the full attention and resources of those involved.
Explanation: In this context, "the supreme necessity" refers to the highest level of importance and priority given to the need to avoid a two-front war. The phrase suggests that the prospect of fighting a war on two fronts was considered an extremely serious and potentially catastrophic situation that needed to be avoided at all costs.
The use of the word "supreme" emphasizes the critical importance of this issue, and suggests that it was a matter of top priority for the American officials involved in the talks. The phrase implies that the officials believed that the avoidance of a two-front war was more important than other issues that were being discussed during the talks, and that it was their primary focus and concern.