In preparation for the arrival of the migrants, Customs and Border Protection reportedly closed four lanes at the San Ysidro and Otay Mesa ports of entry to install infrastructure.
Mattis said the military was helping border agents with a range of jobs, from ferrying them around by helicopter to putting up razor-wire fencing along the border.
Navojoa sits about 1,200 kilometers (750 miles) from Tijuana, the spot where the migrants plan to cross the border, and more than 3,000 kilometers from the town where they began their journey on October 13: San Pedro Sula, Honduras.
Another 4,000 migrants - the main caravan - are expected to reach the border in a matter of days, fleeing poverty and violence in their home countries.
"All were arrested for trying to cross the border illegally", writes Fox. "We don't want you here!"
More than 2,000 members of the caravan.
Several hundred people from the caravan got off buses and made their way to a shelter on the Mexican side near the border to line up for food.
The developments come as more than 2,000 members of the caravan - which has made global headlines for the past month and added fuel to the already heated immigration debate - are expected to arrive at the border on Thursday.
The first wave of migrants in the caravan, which became a central theme of the recent US election, began arriving in Tijuana in recent days, and their numbers have grown each day.
A few people pitched tents at the Tijuana beach plaza while most, like Henry Salinas, 30, of Honduras, planned to sleep there in the open.
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Various videos show the migrants sitting on top of the fence, and the Associated Press reports that one migrant even jumped over the fence into us territory briefly before climbing back over into the Mexican side as border agents watched from a distance.
Some migrants said they were waiting for others members of the caravan to arrive to figure out their next steps.
This week, hundreds of migrants from a caravan originally starting off in Honduras last month, began arriving to Tijuana as part of the final leg of their journey to reach the US border and claim asylum.
"We have to see what we're offered, just so they don't send us back to our country", said Jairon Sorto, a 22-year-old Honduran who arrived by bus Wednesday. He said he refused to consider Mexico's offer of asylum in the southern part of the country because it was too close to Honduras and he felt unsafe from his country's gangs.
"We're not criminals!" some of them shouted, in a pointed message to US President Donald Trump, who has deployed some 5,900 troops to the border as the caravan approaches.
Visiting the other end of the border - Base Camp Donna, in Texas - Mattis insisted the military deployment was "necessary" to support US Border Patrol, even as some in Trump's own Republican party criticized it as a stunt.
With a total of three caravans moving through Mexico comprising 7,000 to 10,000 migrants in all, questions arose as to how Tijuana would deal with such a huge influx, especially given USA moves to tighten border security and make it harder to claim asylum.
The Central Americans in the caravan are the latest migrants to arrive in Tijuana with the hope of crossing into the United States. By early Wednesday, an additional 1,100 migrants from a third and last caravan also arrived at the stadium.
Numerous migrants in Tijuana say they will wait for the rest of the caravan to arrive before trying to cross.
Read the full report at Fox News.