The Halifax Explosion on December 6, 1917 occurred at 9:04:35 am (noted in the seismographic recordings from the time) and until 1945 was the largest man-made explosion ever.
A maritime accident caused by the misunderstanding of pilot signals and involving the collision of two slow moving ships in the harbor, it killed thousands, flattened the city of Halifax, caused a tsunami that drowned many of the victims and started fires in parts of the city that hadn’t been flattened. Telegraph messages notified Boston and the rest of Canada of the calamity that had occurred. Being the closest major city, Boston responded immediately. Within 13 hours, Boston sent a train laden with medical personnel, food supplies, shelter and clothing. Because of a snow storm, it didn’t arrive until December 8, but it was the first humanitarian assistance from outside the area, and it was highly appreciated, as you can imagine.
This digital report from the CBC radio show As it Happens, explains how the scientists working on the Manhattan project secretly came to Halifax to understand more about what a ground burst would mean and do. When in college, I wrote a paper on the topic and was reminded of this just two weeks ago, when I read a news report of the goodwill that the province of Nova Scotia sending a Christmas tree to the city of Boston every November seeks to celebrate.
Similarly, the government of The Netherlands sends thousands of tulip bulbs to Ottawa, the capital of Canada, for hosting the Dutch royal family during the second world war.