President Donald Trump left out a key detail this week when he outlined his plans for a massive missile and air defense shield over the continent: He can’t build it without Canada.

Ottawa has historically funded about 40 percent of NORAD investments, according to VanHerck, and is putting $38 billion into the command to add new radars in the north over the next two decades. Without those investments — and additional sensors that can peer over the North Pole — officials believe the U.S. will have trouble putting together a credible North American air defense.