Katherine Yon Ebright at the Brennan Center:
The Alien Enemies Act of 1798 is a wartime authority that allows the president to detain or deport the natives and citizens of an enemy nation. The law permits the president to target these immigrants without a hearing and based only on their country of birth or citizenship. Although the law was enacted to prevent foreign espionage and sabotage in wartime, it can be — and has been — wielded against immigrants who have done nothing wrong, have evinced no signs of disloyalty, and are lawfully present in the United States. It is an overbroad authority that may violate constitutional rights in wartime and is subject to abuse in peacetime.
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The president may invoke the Alien Enemies Act in times of “declared war” or when a foreign government threatens or undertakes an “invasion” or “predatory incursion” against U.S. territory. The Constitution gives Congress, not the president, the power to declare war, so the president must wait for democratic debate and a congressional vote to invoke the Alien Enemies Act based on a declared war. But the president need not wait for Congress to invoke the law based on a threatened or ongoing invasion or predatory incursion. The president has inherent authority to repel these kinds of sudden attacks — an authority that necessarily implies the discretion to decide when an invasion or predatory incursion is underway.
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Today, some anti-immigration politicians and groups urge a non-literal reading of invasion and predatory incursion so that the Alien Enemies Act can be invoked in response to unlawful migration and cross-border narcotics trafficking. These politicians and groups view the Alien Enemies Act as a turbocharged deportation authority. But their proposed reading of the law is at odds with centuries of legislative, presidential, and judicial practice, all of which confirm that the Alien Enemies Act is a wartime authority. Invoking it in peacetime to bypass conventional immigration law would be a staggering abuse.
