By William James and Elizabeth Piper LONDON (Reuters) -Britain warned it might be forced to “act” in a row with the European Union over post-Brexit trade rules for Northern Ireland, raising the risk of commercial strife and deepening a standoff stirring U.S. concerns for peace in the province. Foreign Secretary Liz Truss said on Thursday Britain would have no choice but to act unless the bloc was more flexible in talks over trade rules for the British-run province — one of the most difficult issues in Britain’s exit from the EU. When Britain quit the EU, Prime Minister Boris Johnson’s governm…