Austria sent 2,200 troops to its border with Hungary on Monday to reintroduce border controls, a day after Germany broke with the EU’s open-borders agreement and began requiring valid passports for entry from the overwhelming number of migrants seeking refuge. German officials said the new requirements wouldn’t affect migrants’ ability to seek asylum, but pleaded for assistance from other European countries to cope with the growing humanitarian crisis. Train service between Austria and Germany resumed Monday, after weekend service was shut down. On Saturday alone, more than 13,000 refugees arrived by train from Austria.