Yuliya Panfil is a senior fellow and director of New America's Future of Property Rights program. Previously, she worked at Omidyar Network, USAID and as a journalist. The views expressed here are hers. View more opinion on CNN.

(CNN) President Donald Trump can declare a national emergency, and the government can take land for the border wall. But Trump can't do both.

University of Texas property law professor Christopher Johns can see into the future. His final exam for last year's eminent domain and property law class included the following question:

President Trump has decided to build the Wall, but Congress refuses to fund or consent to his project. Frustrated, Trump decides to go it alone, and acquire land for the wall anyway. Can he do it?

The correct answer? No, the President cannot unilaterally take land without Congressional authorization.

"If my students said 'Yeah, the president has the power to do this,' I wouldn't have given them any points," Johns told me.

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In the days since Donald Trump declared a national emergency on the southern border, he has been rebuffed by Congress and sued by 16 states. Politicians and litigators argue that Trump doesn't have the authority to divert funds for constructing a wall, but they miss a potent argument: Trump may not have the authority to seize the land to build his wall.

In order to build a wall, the government must use its eminent domain power to seize more than 1,000 miles of property along the border. That power to take private property for public use is bedrock to a functioning government: It allows us to build bridges and highways, establish public parks and run utility lines.

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But, eminent domain power rests in the legislative branch . That means the President needs congressional authorization to take land. By going around Congress and declaring a national emergency, Trump just ensured that congressional authorization isn't coming any time soon. In doing so, he may have created a massive barrier -- so to speak -- to his ability to construct the wall. He needs land for the wall, and if he's not allowed to take the land, he's stuck.

Perhaps unsurprisingly, there aren't many court cases that involve a president trying to grab property by fiat. The most cited analogue is the Supreme Court's 1952 decision in Youngstown v. Sawyer . There, at the height of the Korean War, President Truman had attempted to use his executive powers to nationalize steel mills in the face of a labor strike. The Supreme Court said that even during a war the president cannot seize private property without legislative authorization.

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The court rebuffed Truman's attempt to use the Korean War as a justification, stating that "even though 'theater of war' be an expanding concept, we cannot with faithfulness to our constitutional system hold that the Commander in Chief of the Armed Forces has the ultimate power as such to take possession of private property in order to keep labor disputes from stopping production. This is a job for the Nation's lawmakers, not for its military authorities."

So if this argument is such a slam dunk, why aren't we hearing more about it?

One reason may be that property is a highly specialized field of law, and there just aren't that many lawyers who can speak authoritatively on both emergency powers and eminent domain. Of 11 experts that Vox recently asked whether Trump can use emergency powers for the wall, only one -- George Mason University law professor Ilya Somin -- discussed the property rights angle in detail. I reached out to the other 10 experts: Most responded that while Somin's argument sounded right, they didn't know enough about property law to comment.

Another reason may be the counterargument that if we assume Trump has Congressional authorization to fund part of the wall (for example by reprogramming drug interdiction funds), and seizing land is necessary for building that wall, then by extension he's authorized to seize the land. But, it's far from clear whether that causal link holds water, and the question would likely be litigated.

Two things, however, are certain.

The first is that Trump's border wall is making strange bedfellows of human rights-loving liberals and property rights-loving conservatives. Private property rights are a bedrock of conservative politics, and many Republicans are in the squirmy position of choosing between this first principle and Trump's campaign promise. Several Republican members of Congress to come out against the wall, Will Hurd and Justin Amash among them, have done so on property rights grounds.

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And the second: If Trump makes a unilateral land grab, landowners will tie him up in court for years. In fact, that is the most likely outcome of the wall saga. Without congressional authorization, Trump can't move quickly to build his wall. Instead, he'll get stuck in court until 2020, and then who knows what will happen?