MADRID—Spain faced a humanitarian and diplomatic crisis Tuesday after thousands of Moroccans took advantage of relaxed border controls in their nation to swim or paddle in inflatable boats onto European soil. By Tuesday morning, around 6,000 people had crossed the border into the Spanish city of Ceuta since the first arrivals began early Monday, the Spanish government said, including 1,500 thought to be teenagers. The city of 85,000 people lies in North Africa on the Mediterranean Sea, separated from Morocco by a double-wide, 10-meter (32-feet) fence. The sudden influx of migrants has deepened the diplomatic row between Rabat and Madrid in the wake of Spain’s decision to allow in for medical treatment the chief of a militant group that fights for the independence of Western Sahara. Morocco annexed the sprawling nation on the west coast of Africa in 1975. Migrants soaked with seawater still kept reaching Ceuta on Tuesday although …