It certainly complicated by what breed of 'state' S. Vietnam was, but we certainly came to their aid in the context of the North trying to conquer them (or France's depending on how you take the history) MB notes Kuwait. We took a long time getting to it, but we did enter WWI without being attacked directly - Lusitania not withstanding. WWII would not count as we sat until we were attacked ourselves. There were a lot of Americans flying in SE Asia before Pearl Harbor, but it wasn't official.