Um, the preamble to the Constitution is not law. You need to read a little farther than the preamble, specifically to Article 1, Section 8, Clauses 1 and 11.
Using your line of thinking, where in the constitution did it give us the ability to go to war with Germany? They didn't attack us and many countries have declared war on us without us reciprocating.
They didn't attack us? I think you need to study the history leading up to WWII just a wee bit. German U-boats were sinking US flagged merchant vessels who were supporting Great Britain's war effort long before we declared war on Germany. This was an act of war on the United States. Civil war in Libya is not an act of war against the United States regardless of how much trade we do with the region (which isn't much outside of us buying oil from them that we don't need)