(CNN) It's part activist march, part humanitarian mission.

The images are striking: throngs of Central American migrants crossing Mexico during Holy Week as they make their way north to the US border.

It's become an annual tradition, and getting publicity has always been a goal for this and other so-called "caravans" that periodically make their way through Mexico.

Mexico has the absolute power not to let these large "Caravans" of people enter their country. They must stop them at their Northern Border, which they can do because their border laws work, not allow them to pass through into our country, which has no effective border laws..... — Donald J. Trump (@realDonaldTrump) April 2, 2018

Trump's tweets place the caravan at the center of the US immigration debate, suggesting that the latest waves of people are coming to the United States because of lax laws that don't do enough to crack down on illegal immigration.