Homeland Security Secretary Kristjen Nielsen’s resignation early this week had tongues wagging all over Washington and in the news media. Why did she step down? Personal differences between her and President Donald Trump? One of those power struggles among White House aides? Frustration that her agency has been unable to curb the flow of illegal immigrants into the United States?

Perhaps Nielsen will tell us, at some point.

Reports Sunday and Monday were that Nielsen was not forced out of her job, but left of her own volition. Her timing, submitting her resignation after a Sunday meeting with the president, suggests she did not get what she wanted out of the conversation.

Do she and Trump — and his aides — disagree over tactics or strategy in keeping the nation’s borders secure?

That certainly is possible. But another factor may have been in play.

Despite what some such as House Speaker Nancy Pelosi maintain, the situation on the border with Mexico has reached the status of an emergency. Border Patrol agents stopped or turned back an estimated 100,000 illegal immigrants in March alone. No one can say how many people made it into the United States without being intercepted — though the number must be very large.

Attempting to keep the border secure has strained Homeland Security resources. For example, the department has facilities adequate to house only about 3,000 families detained after trying to sneak across the border. Nielsen had asked the Department of Defense for help in providing more housing.

During recent months, she has complained about the failure of other federal agencies to assist her department in coping with the swell of illegal immigrants. Was that discussed during her meeting Sunday?

If that was among the frustrations that drove Nielsen out of office, the White House should address it with her successor.

Illegal immigration is a concern too big, too serious for the usual turf-control games to be played among the bureaucrats. If Homeland Security officials need help that can be provided by other agencies, it should be forthcoming — immediately.