When a country applies the same tariff to all nations, it will always import from the most efficient producer, since the more efficient nation will provide the goods at a lower price. With the establishment of a different forms of Regional economic integration, the same country either reduces or eliminates all the trade barriers for the member countries participaing in the agreement. And if the agreement is signed with less cost efficient nations then their products become more cheaper in those importing countries which are the part of this agreement. So, the Importing nations starts to import products from those countries which have less competative advantage rather than those countries which have more competative advantage. Or in other words, after the establishment of the agreement, the importing country would acquire products from a higher-cost producer, instead of the low-cost producer from which it was importing until then. Eventually, this would cause a trade diversion.