The North America Free Trade Agreement, or NAFTA, underpins $1.2tn of trade between the US, Canada and Mexico. President Trump says it’s unfair to the US and has forced its partners to renegotiate the deal. Amid threats of tariffs and exclusions, he wrapped up an agreement with Mexico last week, and this week Canada is back at the negotiating table. But Ottawa seems determined not to back down on key issues. The Trump administration wants to scrap a dispute-resolution mechanism that Canadian prime minister Justin Trudeau says is crucial. The two sides are also arguing over US demands for more access to Canada’s closed dairy market. The BBC’s Susannah Streeter spoke with US economist Irwin Stelzer of the Hudson Institute about the tactics President Trump has been using while renegotiating the NAFTA deal.