The 1839 was the justification for going to war, but there's evidence that Britain was gearing up for war. Look at the statistics for the build-up of armaments, especially shipping. It's also interesting to look at the tone of newspaper articles in the few years before 1914. They were more often or not full of jingoistic nationalism. Of course, there were people who didn't support war, including pacifists, but they were generally rubbished by the establishment. The foot soldiers (and sailors), who made up the majority who died. didn't matter to the "powers that be" playing their military and imperial games.