By the time improved American air and naval defences had driven German submarines from American shores, 5,000 Allied sailors had been killed for negligible losses in U-boats. Dönitz ordered simultaneous operations in the Caribbean Sea. The ensuing Battle of the Caribbean resulted in immediate dividends for U-boats. In a short time, at least 100 transports had been destroyed or sunk. The sinkings damaged inter-island trade substantially. Operation Neuland was among the most damaging naval campaigns in the region. Oil refinery production in region declined while the tanker fleet suffered losses of up to ten percent within twenty-four hours. However, ultimately Dönitz could not hope to sink more ships than American industry could build, so he targeted the tanker fleet in the Caribbean and Gulf of Mexico in the hope that depleted oil transport would paralyze shipyard output. 33 transports were sunk in July before Dönitz lost his first crew. The USN introduced effective convoy systems thereafter, ending the "carnage."