The 1890s were a rare break in great power conflict, but they were only a few decades after the civil war, Imperialism was in full swing, and there were still plenty of wars. In general, there has not been a more peaceful time than the present.
https://en.wikipedia.org/wiki/Napoleonic_Wars
"The Napoleonic Wars (1803–1815) were a series of major global conflicts pitting the French Empire and its allies, led by Napoleon I, against a fluctuating array of European powers formed into various coalitions."
https://en.wikipedia.org/wiki/Seven_Years%27_War
"The Seven Years' War (1756–1763) is widely considered to be the first global conflict in history, and was a struggle for world supremacy between Great Britain and France."
So the smiles could just be the effect of them not knowing they were being photographed.