Donald J. Trump made many sweeping statements about his plans to change the U.S. immigration systems when he was running for office--statements that led some Americans to vote for him, and others to fear what lies ahead.
However, Mr. Trump is not a lawyer, and may not realize the limits on his powers as a U.S. president. Let's take a look at how far these powers do and don't extend.
The first thing to understand is that U.S. immigration law is federal, meaning it is enacted by Congress, signed by the president, and then carried out by the Department of Homeland Security (DHS) and Department of State (DOS). The DHS and DOS follow up by enacting regulations explaining how they will interpret and carry out these laws. What's more, the various agencies that help administer the laws, including U.S. Citizenship and Immigration Services (USCIS, a part of DHS), draft various policies to govern their work.
The bottom line is that the president cannot unilaterally change U.S. immigration law. The primary ways to actually make significant changes in this area are as follows.
Issuing an Executive Order
As President Obama did in instituting the Deferred Action for Childhood Arrivals (DACA) program, the president has some limited discretionary power to guide federal agencies and direct their enforcement priorities. With DACA, for example, President Obama could not have legally ordered that the recipients receive green cards. Instead, he shifted DHS's enforcement priorities, ordering that it refrain from deporting a select group of students (while continuing its agency priority of deporting criminal immigrants with few ties to the U.S.), and that USCIS issue work permits to those who qualified.
President Trump will certainly have the power to undo this order, and to issue other orders directing enforcement activities toward or away from various groups of immigrants. But let's not forget that he also has to work within limited budgetary resources. No president yet, regardless of intention, has been able to obtain approval from Congress to allocate the resources it would take to remove the millions of undocumented immigrants that Donald Trump is proposing to deport.
Amending Immigration Regulations
The president has some indirect ability to change or add to the regulations by which the various immigration-related agencies operate. He will have chosen the head of DHS, and can assume that this person will cooperate in initiating changes to the Code of Federal Regulations.
These changes do not require approval by Congress. They do, however, require the agency in question to formally propose the new regulations, open them to formal public comment during a period of several weeks, consider the comments, and finally issue a final version. Like anything bureaucratic, this process can take a long time.
Push for Changes to Federal Immigration Law
The president cannot make unilateral changes to U.S. immigration statutes or laws. Any changes must first be proposed to, debated by, and passed through with approval by both houses of Congress; the House of Representatives and the Senate. After that, they go to the President for signature (or veto).
It is certainly possible that President Trump will encourage the all-Republican Congress to make major changes to U.S. immigration law, but not overnight, and not without a good deal of public attention.