ARLINGTON, United States — The United States has requested a drastic weakening of the systems that enforce the North American Free Trade Agreement by settling disputes, sources said Saturday of the latest American proposals that would drastically overhaul NAFTA. The U.S. wants to strip down the three sections that settle disputes: Chapter 11 that lets companies sue governments, Chapter 19 that allows companies to fight to overturn duties, and Chapter 20 on country-to-country disputes. The American proposals would render all of them toothless. …The biggest anticipated fight is over Chapter 19. Canada walked out of the original free-trade negotiations with the U.S. in 1987, refusing any agreement without a mechanism that allows companies to fight anti-dumping duties. Over time, Canadian companies have used it in softwood-lumber cases, and could use it again if the Bombardier-versus-Boeing dispute drags on.