First off, I believe neither the War Power Acts nor Congressional Defense Authorizations are sufficient to deal with this problem either in a diplomatic, military or legal sense. Continuing with the current method will not lend any credibility to military and foreign policy. We need a national debate that will set long term military and diplomatic policies regarding terrorism and pre-emptive use of force. I do want to begin this discussion by saying that in the case of an imminent threat, when there is sufficient actionable intelligence, I of course advocate the President using all available resources to thwart an attack be it terrorist or otherwise. In the case where the foreign government is either hostile or suffers from security leaks that could compromise American lives and successful execution, diplomacy would have to wait.
If in the situation where an act of terrorism is initiated by an independent, unaffiliated terrorist or terrorist group, I think the first goal is to work diplomatically and allow the host nation to take care of business quietly and behind the scenes. If they will not I believe the President has the statutory and Constitutional authority to initiate a surgical operation using the CIA and/or Military Special Operations.
In the case of the Taliban, Pakistan, Syria, Somalia etc. whose governments that are either actively supporting or tacitly providing safe harbors towards terrorist networks or affiliated terrorist groups such as Al Qaeda should be put on notice by the United States by a formal Declaration of War against the terrorist organization. This allows a full and public debate in Congress imposing strict accountability of those in favor and against, there will be a greater likelihood of diplomatic means being exhausted and far more consistency in policy both by the initiating President as well as future Presidents. No more subjective policing actions. Korea, Vietnam, Gulf War I, Somalia, Balkans, Gulf War II and the War on Terror were unconstitutional wars and never should have been allowed to happen in the manner they did.
The ability to engage in military action for extended engagements (note the difference between the above actions and actions in Grenada, Panama Canal and Libya) has been abused in a bipartisan manner by Truman, Kennedy, Johnson, Nixon, Bush, Clinton and Bush all relying on UN Security Counsel Resolutions. It’s a ceding of Congressional authority allowing an Executive power grab that is been useful for Presidents seeking politically expedient military quick fixes and useful for Congressmen who have sought plausible deniability allowing them to vote for something before they vote against it. Clearly defined Congressional oversight and an end to ad-hoc diplomatic, foreign and national security policy is the third rail answer to the question.