Al Queda did declare war on the US in 1997
We only paid attention to their declaration after 9-11. Since they declared war on us, we
They're a multinational outfit, so we have to go where ever they happen to be. International terrorism is far larger than Al Queda alone. To fixate on one organization is to lose sight of the bigger threat.
In the end, the US has only formally declared war on another country a handful of times in our history, but we've fought many times. We never declared war on Korea or Vietnam, either, but thousands of Americans fought and died there.