What were "dum-dum bullets" and why were they banned in 1908?

In the field of firearms, an expanding bullet is a bullet designed to expand on impact. Such bullets are often known as Dum-dum or dumdum bullets.

The Hague Convention of 1899, Declaration III, prohibits the use in warfare of bullets which easily expand or flatten in the body, and was an expansion of the Declaration of St. Petersburg in 1868, which banned exploding projectiles of less than 400 grams.


https://en.wikipedia.org/wiki/Dum-dum_bullets

Dum-Dum bullets are made to expand upon impact and tear organs and body tissue. They were considered barbaric by the International Peace Conference, and banned from use in 1899.