What is the difference between a declaration of war and an AUMF?

Is it just the wording that “a state of war exists with x” versus “the President is hereby authorized to use military force against x”?

There are specific laws which are triggered by a declaration of war, but not an authorization for the use of military force. For example, under a AUMF, there is a limit on the number of reservists that may be called up and for how long. Under a declaration of war, there is no limit.

In terms of satisfying the Constitution, there is no difference, despite the protestations of some literalists. Congress passed a couple of AUMFs before the first declaration of war against Britain in the War of 1812, so the often-heard criticism that AUMFs are some kind of new development to substitute for a declaration of war are categorically false.

What is it about an AUMF that doesn’t trigger the same laws as a declaration of war. Is it purely textual within the resolution?

No, it is how the other laws are written. For example, let’s take something that has been in the news. 50 USC 1811 says: “Notwithstanding any other law, the President, through the Attorney General, may authorize electronic surveillance without a court order under this subchapter to acquire foreign intelligence information for a period not to exceed fifteen calendar days following a declaration of war by the Congress.”

So, within 15 days of Congress passing a declaration of war, the President may authorize warrantless wiretapping. It doesn’t say that the President may do that after Congress passes an AUMF.

There are also differences in terms of eligibility for medals, combat pay, DVA support, G.I. Bill eligibility, etc. for military personnel engaged in fighting under a declaration of war as opposed to an AUMF. But Congress often retroactively recognizes conflicts in which war was never actually declared (such as Korea and Vietnam) as legally tantamount to war, with all the benefits to the warrior that ought to accrue, as since 1941 we’ve kinda gotten out of the habit of declaring war.

Yes but what makes an AUMF not a declaration of war? To me, an AUMF seems like simply a conditional declaration of war.

Just the way that it is phrased. An AUMF will say something like, “The President is authorized to use the Armed Forces of the United States to do such and such.” A declaration of war uses terms like, “Congress recognizes that a state of war exists with respect to Fredonia.”

There are conditional AUMFs. There’s still a law on the books that if a Middle Eastern nation requests help in defending itself from a country controlled by “international Communism,” Congress has already authorized the President to take action. That law was passed in 1957 and is still valid law today.

In most ways, the end effect of a declaration or AUMF is the same – the President gets to bomb another country. But the additional powers that come with the power to invade are the difference.

On the receiving end (e.g. Iraq), there is not much difference.

The differences all lie within what changes (powers granted, etc.) domestically.

Beyond statutory law, there are a whole gamut of powers invested in the President by Article II as traditionally interpreted, collectively known as “the War Powers.” Again an issue of interpretation and Presidential custom, but only some of these are triggered by an AUMF as opposed to a Congressional declaration of war.

Is there a standing authorization for emergency use of force, or is emergency action considered to be an implicit power of the President?

Emergency action in self defense to an actual attack on the US is universally held to be an implied power of the President as Commander in Chief.

Emergency action beyond that has been subject to considerable debate. Currently, the War Powers Resolution allows the President to deploy troops into hostilities for a limited amount of time without congressional authorization. Beyond those time limits, troops are supposed to be removed unless Congress authorizes the use of force. The War Powers Resolution is controversial, as every president since it has passed has objected to its constitutionality.