"It's what you don't see coming that hits you the hardest"
People were known to have the same attitude about the possibility of international warfare prior to Second World War. Treaty of Versailles was meant to be a "peace treaty" between European nations after First World War (Germany and Allied Nations). People would have expected state leaders of Germany to adhere to the "peace treaty" because it was by Allied Nations in effort to restore peaceful relations with the oppositional forces of WW1. But when Germany started to annex Eastern European nations in the late 1930's (e.g. Austria, Czech Republic & Poland), it started to become clearer to world observers that a new full-scale war between Allied Nations and Germany was due to erupt
Today, we are seeing the same kind of behaviour exercised by superpower nations in the Eastern world. Almost a year ago, Russia attempted to annex Ukraine, which their efforts are still underway. Plus there's the fear that China will eventually attempt to annex Taiwan, a country that happens to be one of the largest semiconductor chip producers on the planet (and mostly produces for US-based companies). Plus there's North Korea, an authoritarian state nation and nuclear superpower, next to South Korea, a nation of liberalized rice & noodles supported by USA.
History has shown that annexation of countries is one of the main factors which contribute to full-scale warfare between nations
As for the Pacific War, it seems the United States did not anticipate a full-scale conflict with Imperial Japan.
The attack on Pearl Harbour was said to have taken the USA by surprise.
In fact, it was said the only tensions held between USA and Japan prior to Pacific War was economic competiton for resources in the Chinese market. Even while the Japanese were at war with China and killing / raping Chinese civillians, US-based companies continued to conduct business with Japan in metal trade
On December 7, 1941, Japan staged a surprise attack on Pearl Harbor, decimating the US Pacific Fleet. When Germany and Italy declared war on the United States days later, America found itself in a global war.
www.nationalww2museum.org
The story of Pacific War shows just how easily full-scale war can erupt. Imperial Japan was one of the world's most powerful nations before end of WW2.
No one has to anticipate a full-scale war before it happens. All it takes is one or more people in position of power to become cocky enough to think they have the means to successfully colonize or demilitarize another country (like Russians have recently tried with Ukraine). If a state nation believes they have the capacity to colonize or demilitarize another country, and also believe there's a tactical advantage for them to gain from doing so, they will do it
History tends to repeat itself, because people don't learn or they forget over time. Humans have a tendency to repeat the same mistakes, due to negligent practices (which can be the result of poor memory / poor training), and allowing their egos to influence their choice of action