The "War on Terrorism" is unconstitutional, because terrorism is a global phenomenon, basically global mass murder. The Constitution clearly states that you cannot go to war without a declaration from Congress, and declaring war on an idea, an action, is to declare war on every country in the world.
Now, Drone strikes specifically are just as bad as flying a manned aircraft into enemy space, which is a violation of U.N law.
Proponents of drone strikes often cite terrorism is a reason to kill civilians and break international law. "9/11", "9/11", they cry. How would they react if Vietnam decided to drone strike us after the my lai massacre? I doubt they'd be so enthusiastic. Drone strikes aren't bad in and of themselves. In a battlefield, all goes. However when you drop a bomb on say a wedding with 200+ people with women and children in it, just because one of your "sources" state a terrorist MAY be present there, is fundamentally immoral, illegal and just wrong. I don't want my tax dollars stained with the blood of Pakistani children.
The United States government has been using unmanned drones to wreak havoc throughout the Middle East and elsewhere without regard for the law, because the president, his staff and various military leaders know they don't have to worry about being tried for war crimes in an international court of law. They saw how Bush got away with what he did, and they've decided to go hog wild. In some cases, drone strikes may be legal and justified, if the target of the attack is a known terrorist and the drone actually strikes its target. But as of now, the technology doesn't exist to ensure that that's the case, and it may never exist. We hear about innocent victims of drone strikes entirely too often.
At least, no more so than manned air strikes are illegal. The only difference between a manned flight and a drone is whether the pilot is in the cockpit or in a base, safe from injury some miles away. All in all, they are the same and follow the same chain of command and accountability.
Since 9/11, the United States has been at war with terrorism in general and with al-Qaeda -- to the extent that al-Qaeda still exists -- in particular. In a war, sometimes it's necessary to do whatever it takes to take out an enemy. International law allows for this. Drone strikes are ideal because they put less of our soldiers' lives at risk: we can kill them from here without even going over there.