The most devastating war in recent times was the
Paraguayan War (1864-70) fought between Paraguay and an alliance of Argentina, Brazil and Uruguay. Estimates suggest that somewhere between 60% and 90% of Paraguay's pre-war population died.
In terms of devastating and sudden losses, the Battle of Jutland (31st May 1916) involved the sinking after magazine explosions of three Royal Navy battlecruisers (HMS
Indefatigable, HMS
Queen Mary and HMS
Invincible) and two heavy armoured cruisers (HMS
Black Prince and HMS
Defence), each with 99%+ crew fatalities - 100% in the case of
Black Prince and
Defence. Of their combined crew of more than 5,000, only 26 survived.