More specifically, were the likes of Churchill and Stalin worried about how Truman might conduct post-war talks with Germany, since they were given verbal assurances by FDR that Truman did not know about? Furthermore, were U.S. military leaders, such as Eisenhower, MacArthur and Nimitz, worried about whether Truman would be able to stand up to the personalities of Stalin and Churchill, as well as decisively win the war with Japan?
I ask this because I've read some historical accounts that suggest that George Marshall was very skeptical about Truman's ability to make the "tough decisions" that were required to win the war against Japan. And also, the only two Vice Presidents to serve less time as VP before becoming President (Tyler and Johnson) are generally held in low regard by historians.
I would read “The Accidental President” by A.J. Baime (it maybe free on amazon prime). It talks about the 4 month transition that Truman faced. It also discusses how we felt uninformed but felt pushed to make effective decisions quickly.
As for the relations with other leaders, the main challenge was with the Soviet Union as the relationship was rapidly deteriorating. However, several months before FDR’s death was the Yalta conference. This established a general understanding of next steps including the Soviet Union joining the eastern front, plans on taking Germany, and governing countries like Poland. While Truman knew of the meeting, he was unaware of the exact conversations. Within the first days of becoming president, Truman was made aware of these discussions and committed he would stay true to the words of FDR in both domestic and international fronts.
For a shorter answer, on the Western front, Churchill and Stalin we’re confident since it was understood the end was nearing. On the Eastern front, it very much the US leading the fight so there was less opinion