The principal change is that since World War 2, the United Nations has been the body that a country would need to convince that its war was justified and there are very limited reasons to justify a war.

Humans have a long history of war and given its impact on those affected, there is generally a lot of information recorded about the events. It is arguable that a large chunk of human history is the details of the many conflicts that have been waged between groups. 

Why war used to be declared

Before the almost instant news cycle we are used to now, it was not easy to let everyone know about developments. If two rulers found themselves ready to go to war, it would take a good chunk of time to inform its residents that those who can fight would need to travel to join their commanders and that those that supply food and weapons would need time to both produce and transport the goods. Similarly trade and travel between nations that are about to go to war would need to let those who are in the other country some time to leave and to conclude or cancel trade deals. 

All that coordination would result in chaos if claims of war were just rumours, so formal declarations made sense.

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