The US president annexing land seems obviously absurd in the modern world. But, it does have at least some legal precedent. For instance, the purchase of Alaska or the Louisiana Purchase heavily involved negotiations on the side of the executive branch.

Could the US president actually annex land if they wanted to?

For example: say that the Mexican drug cartels become so powerful in Mexico that they effectively are acting as their own fully-formed states. Congress might be unwilling to solve the problem. Using this as a pretext, an expansionist president could send in the army, could declare that northern Mexico belongs to the United States, and could grant citizenship to all the people who lived on the annexed land without the approval of congress.

Could the US president actually do this?

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    How does "unilaterally annex territory" differ from "invade a foreign nation"? The President could order the army to invade Canada or Mexico as it did during the Mexican-American War (1846-1848) history.com/topics/19th-century/mexican-american-war Jan 28 at 6:40
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    "Like this" meaning: purchase more territory? Declaratively annex something? Invade? Jan 28 at 15:09
  • @JustinCave It differs, because the invasion is a necessary precondition, that may or may not be followed by annexation.
    – gerrit
    Jan 28 at 17:42
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    @JustinCave "unilaterally annex territory" differs from "invade a foreign nation" on several counts. Most notably, one phrase includes the adverb "unilaterally" while the other does not. Theoretically, at least, the US president does not have the power to wage war unilaterally, and while there is less clarity around that as undeclared wars have become common, congress would certainly get involved if the president attempted an unauthorized military action of that scale. Second, invasion doesn't imply annexation; it could just be occupation, whether for a short or long term.
    – phoog
    Jan 28 at 21:41
  • I was thinking more of a scenario where drug cartels in Mexico get more and more powerful in the future, to the point of effectively acting as their own states. Said US president with expansionist views points to the inability of Congress to solve the problem and sends in the military, annexes mostly desert areas of the Mexican border, and grants citizenship to those Mexicans living in those areas. Regardless of the obvious extremity of such a scenario, is it possible on legal grounds? Jan 29 at 3:09

3 Answers 3


A president could certainly negotiate the purchase of territory in Canada and Mexico. However, buying territory requires money, and money comes from taxation, which is controlled by Congress.

The President could direct the US Army to attack Canada and Mexico, but unless this is in response to some emerging military situation, this looks very much like a de facto declaration of war. Only Congress can declare war. And again, wars need to be paid for (soldiers and munitions don't come for free), and Congress controls the purse.

The President could sign a piece of paper that declares Canada to be a territory of the USA, but it isn't a law, neither in Canada nor the US. And it isn't self-enforcing. It would require action, and action costs money.

Of course, in customary international law, invasions of peaceful neighbours are forbidden (which doesn't mean it never happens).

The President could not admit Canada and Mexico as new States of the USA; that is an explicit prerogative of Congress.

The question of granting citizenship is muddier. There is no precedent for the President to do this. But it is clear that citizenship could be granted by Congress. It seems unlikely that the President could grant the full rights of citizenship without at very least the tacit approval of Congress. But the President could well order that there be no border control or order that US passports be issued.

If Congress did not approve of any part of an executive order, it can overrule it by legislation. And Congress can also remove the President, so it is very hard for the President to act without the tacit approval of at least part of Congress.

Finally, I've not mentioned that Canada and Mexico might very well have some opinions about being annexed by the USA. Their respective constitutions do not allow for the government to sell the entire country. And they are likely to resist with extreme force any invading army.

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    The one scenario this doesn't address is the one where part of Canada or Mexico decides to secede in order to join the US, and whether the President could unilaterally accept them as US non-state territory. Of course, that would probably mean taking sides in their resulting civil war, but I can think of scenarios where that could be avoided.
    – Bobson
    Jan 28 at 11:46
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    @Bobson Although it's not crystal clear, "The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States..." suggests to me that Congress alone has the authority to acquire new territories as well.
    – Brian
    Jan 28 at 18:54
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    @Bobson - West Florida, Texas, Hawaii, etc... every time land is annexed by the US, Congress is involved by ratifying a treaty or a joint resolution to annex the land. Jan 28 at 22:31
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    The US has invaded lots of places without congressional approval. The question is vague about what annex means, and some forms of incorporation are certainly outside his power, but he could probably invade and establish military rule, citing security needs and Presidential authority over the military - especially if the invasion was quick. Whether such a place would be part of the US per international law is dubious, but as the Israeli/Palestinian Occupied Territories show, that may not matter.
    – Stuart F
    Jan 30 at 11:10

The State Department rather recently announced the extent of the US continental shelf. So, for some declarative purposes at least, they think that power resides with the Executive. True, that decision was made considering the output of an "ECS Task Force", but the latter doesn't mention being set up as a result of some law, so it was purely an executive decision, it seems. Previous declarations in this regard were also made by the executive (Truman).

But it if came to other matters like sending in an army or introducing a new state to the Union that would probably require some Congress participation. (Sending an army might depend on how some standing AUMF might be interpreted, as well.) I'm less sure about the extent of US Territories, e.g. if the President could unilaterally (even artificially) extend one of those to cover some new area. This is somewhat complicated because of the legal division on those, which include the notion of "unincorporated, unorganized" territories, which e.g. American Samoa seems to be one (remaining):

technically considered "unorganized" since the U.S. Congress has not passed an Organic Act for the territory.

OTOH Congress does explicitly send some money over there

Federal dollars enter the [American Samoa] economy through congressional appropriations, categorical grants [...]

They also have a House delegate, although Wikipedia doesn't mention (on that page) when that first happened, although another page says 1970, while the territory was acquired some ~71 years before (1899).

The formal treaties that ceded the islands [1900-1904--there were two] were ratified by Congress in 1929. So at least for practical purposes, a signed but unratified treaty can serve as basis for US [troop] presence for a considerable amount of time. It's also worth noting though that Congress passed some appropriations as far back as 1889 "for the protection of American lives and property in Samoa".

  • FWIW, residents of U.S. military bases abroad under exclusive U.S. jurisdiction, Thomas v. Lynch, 796 F.3d 535 (5th 2015), residents of American Samoa, Tuaua v. U.S., 788 F.3d 300 (D.C. Cir. 2015), and residents of the Philippines when it was a U.S. territory, Miller v. Christopher, 870 F.Supp. 1 (D.D.C. 1994), affirmed 523 U.S. 420, do not gain citizenship at birth in the United States by virtue of Section 1 of the 14th Amendment, under current case law.
    – ohwilleke
    Jan 30 at 20:21

Hmmm, from a very high level, isn't that mostly what Polk did in Texas in 1845-1848, taking chunks of land from Mexico?

In July, 1845, Polk, who had been elected on a platform of expansionism, ordered the commander of the U.S. Army in Texas, Zachary Taylor, to move his forces into the disputed lands that lay between the Nueces and Rio Grande rivers. In November, Polk dispatched Congressman John Slidell to Mexico with instructions to negotiate the purchase of the disputed areas along the Texas-Mexican border, and the territory comprising the present-day states of New Mexico and California.

Following the failure of Slidell’s mission in May 1846, Polk used news of skirmishes inside disputed territory between Mexican troops and Taylor’s army to gain Congressional support for a declaration of war against Mexico. On May 13, 1846, the United States declared war on Mexico.

There seems to have been some resistance at a congressional level, but pre Wars Powers Resolution a POTUS could, and still can, be up to a fair bit of gunboat diplomacy. Maybe a POTUS can't unilaterally effect an annexation, but they sure as heck could get the ball rolling.

And what are the constitutional arrangements around negotiating a ceasefire, rather than a treaty? Does Congress automatically get a say-so? Or does cessation of combat fall under the mandate of POTUS, as Commander in Chief? What conditions can be attached by the winning, US, side on a ceasefire?

Would it fly nowadays? That is less sure. But wars that end up in cessation of territories have happened. Would Congress then say, "no, they attacked us (the usual justification for these things) but give them their lands back"?. The question's outcome might be less stopped by formal US legal blocks than the dismal prospects of getting the world to go along. And, one hopes, repulsion by the US public as well.

We had several questions after January 6th here, where some people put forth the proposition that it was less the exact laws that stymied Trump's attempt to usurp the election as it was the customs and expectations of the US polity and public, built up over centuries, that heavily favored proper democracy.

I'd like to think the US in 2024 wouldn't behave like it did in 1845. (Or, cough, 1898 Cuba, cough, Philippines, cough.) But it seems more a change in public attitude and international law than based on constitutional grounds which are not very different now from what they were then (the feeble-ish Wars Powers Resolution aside).

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    I would distinguish between occupation of another country and annexation of it, which would make it legally and official part of the U.S. No U.S. law says that land is part of the U.S. just because the U.S. controls or occupies it.
    – ohwilleke
    Jan 30 at 20:27

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