Kirstjen Nielsen has stepped down at Department of Homeland Security. The left are screaming fired, resigned but its more basic that that.
Trump and most businessmen change staff in high pressure positions so they don’t burn out and bad things happen. He has done this with several people Mattis, Sessions, Tillerson, etc. It is normal in business and you did vote for a businessman not a politician, lawyer, or general.
The DHS is at a new stage with both the wall and the caravan. 
I’m an Australian with political interest and a geography degrees, so I have studied borders where the two subjects meet. Australia has been though the debate America is now having. In our case it was stopping the boat loads of refugees and illegals coming across the Arafura sea between Indonesia and Australia. With the bonus challenge of sorting the two out on arrival. 
The problem compounded by the fact that people smugglers told people to destroy their identity papers to force Australia to take and hold them as new ones were created. That will be happening with the caravan; some of the same Soros funded organizations are involved. Some source countries and the UN pulled a go slow on that reissuing ID process. Some countries refuse to take people back. Some people, deemed not a threat but not identifiable have been allowed into Australia. 
This is not a new problem. Millions in WW2 lost their ID. Bombed, confiscated by the Nazis, never acquired lest the Nazi’s know the child exists or in several thousand cases the person is too young or too traumatized to know their real name. A solution was found. The red cross and the UN created an ID system (its been abused more recently so Trump and others are not trusting it). Some today are finding out their real names for the first time in their late 70’s thanks to DNA.
Australia created temporary protection visa’s, proper tracking [we are a smaller country so that is easier], we created off shore processing with people placed in Nauru and New Guinea. Some are still there both willingly and involuntarily.
Technically Ellis island is off shore processing. The processing was done on the ships before they left port. The Ellis Island check was so quick and short because they were only checking that the ships first officer and ships doctor had done their job. If you had a disease or failed a criminal check the whole ship load of people was held at the shipping company’s expense. The same applied to railway companies on both US land borders. You did not get on the border train without a check at the station. Crossing by road was rare and crossing the desert on a horse was dangerous. Today with 4 wheel drives and GPS its easy.
Prep for the wall is done, land purchases made. There are people claiming to have received foreclosure notices but some are in places where there is already a wall?!? I suspect that there is some Soros organization posting out phony foreclosure notices to provoke people. Most of the wall is on land that the government owns or was foreclosed on in the 1970’s. There are people living between the wall and the border in some places and that’s a problem for some. There are also some people who have the border running though their property. Rivers make bad borders because rivers move. One day your whole farm is in Mexico the next after a flood your north paddock is in the USA. There are laws to fix this with no change of nationality or ownership.
They need to implement a new real method of dealing with asylum claims. That requires new court proceedings, lie detection technology and bio-metric tracking. Trump is appointing more than a hundred new judges to do that.
There is new lie detector technology. No lies MRI. When you lie you use part of your brain that you do not use when your are telling the truth or confused. The first generation of this technology is in the capital and several big cities and I believe Gitmo. These are big bulky medical machines. 
The next generation will be about the size of a 1950’s big bulky hair dryer. The key is to ask the right questions and asking “are you a terrorist” will not work. An Islamist will genuinely believe the guy in the F 18 or the drone operator is “the Terrorist”. No lies MRI is not likely to be useful in courts, its self incrimination, but it will eliminate a lot of low risk people. 
We also need a new way to deal with legal refugee claims. The current list leaves out some large classes of refugee claims. That’s an international problem. Most public servants in immigration are atheists and so have little grasp of the difference between Arabic christian and Muslim. They put Sunnis and Shiite in the same accommodation and wonder why there’s a fight. Millions believe in climate refugees, which is another leftist lie. Christians fleeing Islam is pretty basic. However how do you prove they really are Christian? Are you going to say it though a translator wearing a hijab? Etc. More work to be done. 
With south American refugees you have a confusing array of refugee claims: Fleeing land disputes; drug war flight; fleeing communist faction X; tribal wars; conflict between Santa Muerte religious factions; protestants being persecuted by either Catholics and Santa Muerte or both.
Many in the caravan are leftists that failed in an attempted violent revolution in central America. In two cases the free market/ land owner right won and the left fled expected persecution. In one case a rival leftist group won and the refugees are fleeing actual persecutions.
This is like the Stalinist faction verses Trotsky faction in the cold war. Trotskyites claimed, accurately, persecution from the Soviet Union but they were still communists. They were let in and they or their children seem to be running most university faculties. We coped with the short term threat of the trotskyites but not so well with the long term consequences.
The welfare honey pot problem will need to be fixed but that is a whole other essay. I have a possible solution.
Kirstjen Nielsen’s job was to restore morale and rebuild the man power and skill set of the border force. The next challenge is technical and needs a border technology person. The legal challenges is Bill Barr’s job.