European leaders said Wednesday that they would formally propose a landmark trade deal with a cluster of South American nations, boosting Europe’s economic partnerships at a time when relations with the U.S. remain strained. The European Commission in Brussels unveiled its formal pact with the Mercosur nations of Brazil, Argentina, Uruguay and Paraguay that has been under discussion for more than 25 years, sending the final text for ratification by the European Union’s 27 member states and...