Regarding Ukraine, Biden was the one who made the decision not to get the US directly involved in the war. Sullivan even had to play devil's advocate in Biden's decision, because he and the CIA chief thought it was a horrible idea to announce to the world that we were committing to zero troops on the ground. One of Biden's biggest beliefs is that "great powers don't bluff," though, so it was important for Biden to clearly communicate the limits of US engagement in the conflict.
He wanted the Ukrainians to know (according to the reporting in the book) that the US would supply and support them, but the cavalry would not be charging over the hill to save the day. If some horrible event happened in Ukraine, he knew that his generals, the media, and the public would look to him and say "well, don't you think now is the time we should get American boots on the ground? Why aren't you acting?"