Buffett's insurance business took a $2.4 billion underwriting loss from the 9/11/2001 terrorist attacks. The insurance industry as a whole paid $33 billion in claims (in 2001 dollars).
Right, you probably wouldn't get terrorism coverage as part of your homeowner's policy. However, terrorism insurance is big business! Berkshire Hathaway has extended coverage to airlines, the World Cup, the Sears Tower, etc.
That's because terrorism wasn't specifically defined as an "act of war" for general risk policies under the underwriting standards of the time. (although anything nuke has been excluded for many years) War requires a nation-state actor. Terrorism is a broader concept.