Trump wants $5 billion for a wall
Congressional leaders failed to strike a deal with the White House on Friday and the government partially shut down non-essential services today.
The Senate refused to sign-off on the House border wall funding bill on Friday and at midnight the wind-down began.
About three-quarters of federal government programs are funded through to Sept 30 next year, but the financing for all others - including the departments of Homeland Security, Justice and Agriculture - expired at midnight.
The shutdown means US federal parks will close and 400,000 essential workers will work without pay while another 380,000 will be put on temporary leave. A lasting shutdown could affect US employment numbers for December.
A new Congress convenes on January 3 with Democrats taking control of the House. That will virtually ensure that Trump never gets funding for his wall.
Last week was the worst one for US stocks in a decade and that was partly due to talk of a government shutdown. However markets have been through similar episodes in the past and this is unlikely to be a major drag, even if it persists.
At the same time, it may be indicating a more-combative President, which could spell trouble in the year ahead.
