During the 2016 campaign, Trump repeatedly promised to build a "big, beautiful wall" across the U.S.-Mexico border that Mexico would pay for entirely. Mexico rejected the idea of providing any funding for a U.S. border wall. In 2018, Trump requested $18 billion in federal funding for some 700 miles of barrier on the border, mostly to replace 654 miles of aging fence built under the Secure Fence Act of 2006. On December 25, 2018, Trump reversed course, suggesting that he might accept 500 to 550 miles of either mostly refurbished barrier (rather than new barriers in locations that did not previously have them) by November 2020. Trump's proposals and public statements on the wall have shifted widely over time, with varied proposals as to the design, material, length, height, and width of a wall.
In September 2018, Congress passed two "minibus" appropriations bills for the fiscal year 2019 federal budget, which began on October 1, 2018. These bills combined five of the 12 regular appropriations bills covering 77% of federal discretionary funding, and included a continuing resolution until December 7 for the remaining agencies. On December 6, Congress passed a second continuing resolution to December 21, to give more time for negotiations on Trump's proposed border wall, which had been delayed due to the death and state funeral of George H. W. Bush.
A Senate Homeland Security appropriations bill, negotiated by both parties and was reported by the committee to the Senate, provided for $1.6 billion for border security, including funds for "approximately 65 miles of pedestrian fencing along the southwest border in the Rio Grande Valley Sector". The bill did not receive a vote on the Senate floor, although House Democratic leader Steny Hoyer indicated that such a proposal could be acceptable to House Democrats. Senate Minority Leader Chuck Schumer (D.-N.Y.) says the Democratic Party will not support $5.7 billion for the border wall. At a press conference before the government shutdown, he notes “The $1.6 billion for border security negotiated by Democrats and Republicans is our position. We believe that is the right way to go. …” 
Beginning of shutdown
Donald Trump meets with Nancy Pelosi and Chuck Schumer on December 11, 2018, stating "I will be the one to shut it down."
Department of State press briefing with Deputy Spokesperson Robert Palladino on December 18, 2018 about the effects of a possible of a shutdown on government functions.
Overlook (San Francisco), closed for the shutdown in December 2018
On December 11, Trump held a televised meeting with Speaker-designee Nancy Pelosi and Senate Minority Leader Chuck Schumer in which he asked them to support $5.7 billion in funding for a border wall. They refused Trump's demands and an argument followed in which Trump said: "I am proud to shut down the government for border security ... I will be the one to shut [the government] down. I'm not going to blame you for it ... I will take the mantle. I will be the one to shut it down." Schumer replied, "We shouldn't shut down the government over a dispute."
Three days later, Politico reported that Trump was willing to sign a bill with no funding for a border wall that delayed a government shutdown into 2019 and the new Congress. On December 18, following a meeting with Trump, Senate Majority Leader Mitch McConnell said that the government would not shut down on December 22 and that Trump was "flexible" over funding for a border wall. Senate Appropriations Committee Chairman Richard Shelby commented that the most likely resolution was a bill that funded the government until early February. Schumer added that his caucus would "very seriously" consider such a bill and Senate Majority Whip John Cornyn said "I don't know anybody on the Hill that wants a shutdown, and I think all the president's advisers are telling him this would not be good."
The next day, the Senate unanimously passed a second continuing resolution (H.R. 695) lasting until February 8, 2019. Pelosi announced that House Democrats would support such a measure, meaning it would pass the House even in the wake of opposition from conservative Republicans. However, on December 20, following increasing criticism from conservative media, pundits, and political figures, Trump reversed his position and declared that he would not sign any funding bill that did not include border wall funding. The House then passed a version of the continuing resolution later that day that added $5 billion for the wall and $8 billion in disaster aid. Negotiations in the Senate did not lead to agreement on passage of a continuing resolution that day. Trump's changing position caused consternation among Senate Republicans. When asked by reporters what the way forward was, retiring Tennessee Senator Bob Corker laughed: "I don't know. Y'all have fun. I'm getting ready to drive to Chattanooga... You can’t make this stuff up."
The shutdown started December 22 and Trump announced that he would cancel his planned trip to Mar-a-Lago for Christmas and stay in Washington, D.C. The meaning of the term "wall" was expected to be an aspect of the negotiations.