The U.S. government can use any of more than 50 reasons, found in Section 237 of the Immigration and Nationality Act (I.N.A.), to order a non-citizen out of the country. This is true whether the person is in the U.S. unlawfully, is staying lawfully with a visa or other status, or already has lawful permanent residence (a green card).
The primary agency in charge of enforcement is U.S. Immigration and Customs Enforcement (I.C.E.), which may, for example, arrest you after an investigation or a workplace raid. But some people also come to the attention of U.S. immigration authorities in other ways, for example after they submit an application (perhaps for adjustment of status, naturalization, or asylum) to U.S. Citizenship and Immigration Services (USCIS).
Most of the grounds of deportability fall into the following categories.
Crime and Security Violations
The most common reason for deportation from the U.S. is a criminal conviction. Not all crimes can get you deported, but many can, such as drug crimes, firearms offenses, helping smuggle other aliens into the U.S., some crimes of "moral turpitude," aggravated felonies, domestic or family violence convictions, human trafficking, and failure to register as a sex offender.
If you're a non-citizen who has been jailed or imprisoned for a crime, it's unlikely that you'll be released at the end of your sentence. ICE can put a "hold" on your release, so that it can pick you up and put you in detention until pending immigration proceedings.
In addition, any terrorist activity, activity to violate U.S. law relating to espionage, sabotage, export prohibitions on goods, technology, or sensitive information, or activity otherwise endangering U.S. public safety or national security, or any activity in opposition to, or attempting to control or overthrow the U.S. government by force, violence, or other unlawful means, is a ground of deportability.
Participation in past acts of torture or extrajudicial killing, commissions of severe violations of religious freedom, and recruitment or use of child soldiers, are also grounds of deportability.
A whole host of immigration-related violations are grounds for deportation. These include being in the U.S. unlawfully (including by overstaying a visa), violating your status (for example, if you enter the U.S. on a student visa but take a full-time job), having been inadmissible upon entry to the United States (for example, you entered on a family visa meant for unmarried children of lawful permanent residents when you were in fact already married), and falsely claiming to be a U.S. citizen.
You can even be deported for failing to advise U.S. immigration authorities of your change of address within ten days of the move.
Fraud or Misstatements on Immigration Paperwork
Another common reason for deportation involves errors with or misstatements with regard to your documentation, such as your application for a green card.
If, for example, you got your green card because you married a U.S. citizen, and if U.S. immigration authorities can prove that your marriage is a sham because you never really intended to establish a life together, this can get you deported. Or even if your marriage was real, but you submitted a false document showing that your assets were greater than they really were, this too would be grounds for deportation.
You Have Options Before Deportation
In most cases, ICE doesn't have the final say in your deportation. If you don't already have an order of deportation on your record, you have a right to defend yourself in immigration court. You may be able to disprove the charges, apply for a waiver (legal forgiveness) allowing you to overcome the ground of deportability, or apply for some form of immigration relief.
You have a constitutional right to have an attorney represent you in immigration court, though you will have to pay for that attorney yourself. An attorney can help you analyze the law in your case, gather evidence, prepare any witnesses and legal briefs to present to the judge, and more. The U.S. government will also be represented by an attorney, who may cross-examine you and your witnesses.