What does defend the Constitution mean?

To defend the Constitution means to protect it from threats and to actively oppose those who would subvert or overthrow the constitutional order; the promise to defend the Constitution is one of the most serious commitments in the Oath of Allegiance. The oath text in 8 CFR section 337.1 commits the new citizen to support and defend the Constitution and laws of the United States "against all enemies, foreign and domestic," and Article VI of the Constitution requires every federal and state public official to take an oath of similar import.

The phrase "foreign and domestic" is broad: it covers both external threats such as foreign invaders, hostile states, and international terrorism, and internal threats such as insurrection, sedition, or treason. To defend the Constitution, in practical terms, includes serving in the armed forces if drafted (or volunteering for military service), bearing arms or performing noncombatant or civilian service when required by law, reporting threats to constitutional order to appropriate authorities, refusing to take part in unconstitutional actions, peacefully challenging unconstitutional laws through the courts, and (more generally) preserving democratic institutions and the rule of law through participation in civic life.

The two military-service clauses of the oath (bearing arms and noncombatant service) are part of this defensive duty; conscientious objectors may take a modified oath under 8 CFR section 337.1(b) that omits those clauses while keeping the broader support-and-defend obligation. Treason, defined in Article III, section 3 of the Constitution as levying war against the United States or adhering to its enemies, is the most serious violation of the duty to defend.