Transcript for The Trump presidency's top 5 pivotal moments of 2018

The United States will not. We. It might bring to camp president trump again pushing his pulse clean it separating children at the border is required by law there is no block. Instead this is a policy greeted by. His administration. Judges. I want ice didn't look totally. It's. There's that Republicans are. Plan and keep families. Together when you ask the parents become. What should happen. The president made it official signing any new tax on imports you'll. It's really an assault America terrace 25% on killed 10% aluminum aging quickly retaliated. Million dollars tariffs on US. Hitting American manufacturers and our. President trump just moments ago revealing Defense Secretary James Mattis is retiring he will be leaving his post in February this is the most. Resignation from Trump's cabinet yeah simply no longer tolerate differences pat this press. President trump has just accepted the resignation of his UN ambassador Nikki Haley there's no personal reason I think that it's just very important for government officials. To understand. When it. Time that flag attorney general Jeff Sessions has now resigned at the request of the president it takes effect immediately sessions infuriating his boss I recusing himself. From the Russian probe two years ago. Watching today will be able to tell your friends family children or grandchildren here watching the history it may not be the right priests. On the global stage I'm standing right there. And president talked just shy away from holding him accountable for Russia's attack. On US election do you hold Russia all accountable or anything in particular yes they do I hope that both countries responsible. He gives it its not Russian eyewitnesses I don't see any reason why it would (%expletive) and (%expletive) I don't see any reason why I wouldn't. What life it wouldn't crash from this president. Almost un who have walked back following. White backlash after that healthy. Press conference the president reading from a carefully written script and I have full faith and support for America is great in. Historic summit the first time ever a sitting US president. As we've North Korea's leader in this case dictator Kim Jong learned the president Bob believes that he is on a path towards peace. This was eight landmark day for him to leaders signed a letter that included a commitment from the US to provide what's being called unique security assurances. And can declaring that the world will see a significant change. From his regime is Max bloom pressure campaign. Brought us to where we are today.

This transcript has been automatically generated and may not be 100% accurate.