I appreciate that, and I tried to say so in the OP, but just as an intellectual exercise, what examples do we have where it really did work and the consequences of failure could reasonably have been expected to be outright war?.
The Argentina and Brazil War that never happened is an excellent example.
In the 1890s both were in an armaments race fueled by mutual suspicions over intentions regarding the region of the Rio de La Plata. English armaments and loans provided a build-up in tensions. At some point a Brazilian commercial ship was aprehended because it carried weapons upriver. The media on both sides of the border were screaming bloody murder, and residents of the border were accusing each other of violence and discrimination. No shot was fired because diplomats from both countries sat down and discussed the case. Both countries chose to believe that the hidden cargo was meant to some private individual in a farm somewhere and the war ended up never happening. For many decades on the mutual suspicions remained but the war is yet to happen.
Chile and Argentina also came very close to war during their military dictatorships of the 70s, 80s, over a territorial dispute in Tierra del Fuego.
Bolivia and Brazil also had a territorial dispute that involved separatist groups, a highly coveted natural resource (rubber), access to the Atlantic through the Amazon and an American chartered company. Troops on both sides. The diplomat that disentangled this situation, in 1903, became a national hero in Brazil, and it was well-deserved.
Brazil has borders with 10 nations, all of them politically unstable and for many decades under the hands of dictators. Brazil never initiated a war against them and worked to prevent wars from happening all over the Continent. The last one in South America ended in 1935.
No war is ever exclusively about those directly involved. The immediate neighbors all have vested interests and nowadays, more than ever, economical interests prevail and distant neighbors and multilateral organizations will try and interfere too. So, behind the scenes, diplomats of many nations work toward an outcome that favors their own nations - this could be the end of the war and the continuation of it. They can also cooperate and, from a neutral point of view, try and work out a solution that both nations will accept. Before any of that becomes public, much of that has already been agreed beforehand. When it finally goes public, it only changes if public opinion demands it (in democratic states, mostly).
Diplomacy can work both ways, also, like nations that prefer to watch their neighbors bleed to death before interfering. This was the behavior of England for the greater part of the 18c. It would only "wake up" if a nation threatened to reach hegemony in Europe.
