Starting on December 22, 2018, the federal government was shut down for 35 days over Trump’s promise to veto a budget that didn’t include nearly $6 billion in funding for a border wall with Mexico.

Trump claimed that most of the 800,000 federal workers affected by the shut down wanted it to continue until the wall received funding.

As his approval rating declined over the shut down, Trump eventually instead declared a state of national emergency in order to attain funding through other means, siphoning billions from the Defense Department, despite originally claiming that Mexico would pay for it. The shut down was the longest in American history.

The Wall Street Journal – U.S. Government Shuts Down Over Border Wall Funding 
Reuters – Backing down, Trump agrees to end shutdown without border wall money
Wikipedia – 2018–2019 United States federal government shutdown