That's the real issue.
The superficial issue is that a couple built a wall three feet into the boundary vista, which was defined by the treaties and by agreement between Canada and the United States in 1908 that it should be kept clear of all obstruction 10 feet to either side of the border. It is a simple proposition: you can't enforce and secure a border if you can't see it. They built a four-foot-high, 85-foot-long reinforced concrete wall three feet into the boundary vista. It was spotted by the RCMP. It was inspected by a border agent from Vancouver, who concluded the wall needed to come down. The material issue is whether the wall stays up.
The bigger issue is that the U.S. Department of Justice under Attorney General Gonzales, concluded that it preferred private property rights to the public domain and said to let the wall stay up, and he ordered the commissioner from the U.S. side to agree to that. He refused. The President fired him.
What's disputed now in court is whether the President had any power to fire him, because he was appointed under the terms of a self-executing treaty. In Canada this is hard to understand, because you don't have self-executing treaties. You have implementing legislation for this treaty that specifies that the commissioner on the Canadian side is a civil servant of Canada. On the U.S. side, it's a self-executing treaty. He's not confirmed by the Senate. He's not an employee of the United States or an employee of the agency.
The President has asserted that this commission is an arm of the White House, that it's an agency of the United States. That's the big issue.