...isn't kidnapping a soldier from another country and holding him for ransom an act of war (well, that and blowing up their civilians, but I guess that is beside the point?

As you elude to, the Palestinian terrorist factions are already at war with Israel, but it's an unconventional war that people in western society have a hardtime getting their arms around. In many senses, we as a collective public, have not adapted our mindset to the changing nature of threats and the very different face of modern warfare.

Anyone clinging to 18th century warfare doctrine would've been slaughtered when they ran into the weaponry of the Civil War. Anyone clinging to 19th century warfare doctrine would've been slaughtered when they ran into the weaponry of World War I and World War II. Anyone clinging to 20th century warfare doctrine will be slaughtered when they run into modern weaponry. The nature of threats and warfare evolves and transforms constantly.

The founding fathers may have found it convienent to remain free from the tangle of foreign alliances, but they also weren't faced with the threats of intercontinental ballistic missiles, weapons of mass destruction, or an international terror nexus. All they had to do was watch the ocean and guard the beach for all intents and purposes, the ocean was the best defense there was.

Today most nations don't carry out open, large-scale warfare for a host of reasons. What many do engage in, is waging war by proxy or using third parties to further policy goals (see Iran's use of Hezbollah, Liberia's support of guerillas in Sierra Leone, Chavez's support of rebel groups in South America). The result is a lower intensity warfare, frequently playing on ethnic or religious differences, insurgencies, urban combat, etc. The enemies of freedom today are forced to operate in the shadows for the most part, or at least attempt to.

In summary, I sincerely dont believe that most Americans understand the nature of today's threats (neither did all of Europe in the 1930s), where anything short of a 9/11 attack would require a near miracle to act against. It will be much harder to muster public will to act against unconventional threats that don't stoke a strong reaction among the American people.