Must we always need a naval incident?

I'm beginning to think that the US gov't always requires a naval incident to begin (or try to begin) a war.

Think about it:

WWII: Pearl Harbour (ok aerial attack on naval)
Vietnam: Tonkin gulf incident
Iran: Persian Gulf incident

Problem is only Pearl harbour seems to have been the real deal:
http://www.commondreams.org/archive/2008/01/12/6335/

So why does the false naval encounter often seem to be in the playbook?