** LEGAL UPDATE **
As explained in Nolo's book "Becoming a U.S. Citizen: A Guide to the Law, Exam, and Interview," it is important, in filling in USCIS Form N-400, to "show that you're loyal to the United States and will fight for it if necessary."
That's why the book also explains that only certain types of people can safely answer "No" to any of Questions 45-50 in Part 12 of the form, which ask:
Most people should simply answer "Yes" to all of these.
Unfortunately, owing to a printing error, the Ninth Edition of this book contains a sample Form N-400 with "X" marks in the "No" column. Please disregard this portion of the sample, and enter the answers that are true in your case; again, that's ideally "yes" to each one.
See an attorney is your answer is actually "No" and if you do not fit any of the categories described next.
If this application is being filled out for a person who is disabled to the point of being unable to understand the Oath Waiver, Question 46 provides a place to indicate that, and it's possible to answer "No" to it.
Or if you are a conscientious objector (CO), meaning that for religious or moral reasons you refuse to take up weapons or join in a war, you can answer "no" to Question 48 and possibly 49, if you also provide proof of your CO status.
Members of a religion that prohibits taking any sort of oath (for example, the Quakers and the Jehovah's Witnesses can answer "no" to Question 47, but must provide a letter from their church or other religious body confirming membership.
To repeat: In most other cases, applicants for naturalized U.S. citizenship must check "Yes" to Questions 45-50 of Form N-400.
Effective Date: September 21, 2019