Some, but not all; a lot of the missteps in the lead-up to WWI, for example, were where two people had a conversation, agreed on something reasonable, then fail

Some, but not all; a lot of the missteps in the lead-up to WWI, for example, were where two people had a conversation, agreed on something reasonable, then failed to communicate it to third parties who needed to be on board. When all of the great powers are in a room (as in the Security Council, which is I think the most necessary part of the UN) everyone gets all at once a feel for the positions of everyone else.