Meanwhile, submarines became more difficult to detect and destroy. Over the last twenty years, the U.S. Navy has debated how effective non-nuclear submarines would be in wartime and whether the U.S. should buy some of these non-nuclear boats itself. This radical proposal is based on two compelling factors. First, the U.S. Navy may not get enough money to maintain a force of 40-50 SSNs (attack subs). Second, the quietness of modern diesel-electric boats puts nuclear subs at a serious disadvantage, especially in coastal waters. With modern passive sensors, a submerged diesel-electric sub is often the best weapon for finding and destroying other diesel-electric boats. While the nuclear sub is the most effective high-seas vessel—especially if you have worldwide responsibilities and these nukes would have to quickly move long distances to reach troubled waters—the diesel-electric boat, operating on batteries in coastal waters, is quieter and harder to find.

39 nations operate 400 diesel-electric subs. Only three of these nations (China, Iran, North Korea) are likely to use their subs against the U.S. or its allies. China has fifty of these boats, Iran has three (plus 25 much smaller mini-subs), and North Korea has 20 (plus 50 much smaller mini-subs). So, the U.S. must worry about 73 diesel-electric subs and 75 mini-subs. But about half the full-size subs are elderly, obsolete, and noisy. The same is true for at least half the mini-subs. That leaves about 36 full-size subs and 40 mini-subs that are a clear threat (though the older subs can be a threat if you get sloppy). That’s a lot of subs, and they make the East Asian coast and the Persian Gulf dangerous places for American warships.

For much of the past decade, the U.S. Navy has tried to gauge how bad the threat is. So, from 2005 to 2007, the United States leased a Swedish sub (Sweden has only five subs in service) and its crew to help American anti-submarine forces better understand what they were up against. This Swedish boat was a "worst-case" scenario, an approach that is preferred for training. The Gotland-class Swedish subs involved are small (1,500 tons, 64.5 meters/200 feet long) and have a crew of only 25. The Gotland was based in San Diego, along with three dozen civilian technicians to help with maintenance.

For many years before the Gotland arrived, the U.S. Navy had trained against Australian diesel-electric subs and often came out second. The Gotland has one advantage over the Australian boats: its AIP system (which lets it stay underwater, silently, for several weeks at a time). Thus, the Gotland is something of a worst case for what American surface ships and submarines might face in a future naval war. None of America's most likely naval opponents (China, North Korea, or Iran) have AIP boats yet, but they do have plenty of diesel-electric subs which, in the hands of skilled crews, can be deadly.

Based on the experience with Australian and Swedish subs, the U.S. Navy has been developing new anti-submarine tactics and equipment. All this is done in secret, obviously. But apparently modern, quiet diesel-electric boats remain a major threat to U.S. surface warships and subs. Meanwhile, potential enemies are building more of their cheaper, higher-quality diesel-electric boats and training crews by having them stalk actual warships (including U.S. ones). Subs are becoming more numerous, while U.S. defenses are limping along because it's technically difficult to find quiet diesel-electric boats in coastal waters.

One reason China wants to keep American naval forces out of its economic zone (which does not bar foreign warships) is to let Chinese diesel-electric subs train without being stalked by American subs, surface ships, and aircraft looking for realistic practice tracking Chinese boats. At the same time, the U.S. Navy has lost full use of its most effective underwater anti-submarine training area (a well-mapped, instrumented area off southern California) because environmentalist activists have convinced judges that active sonar in this training area harms some aquatic species.

Moreover, the North Korean and Iranian fleets (and governments) are in decline, while China is pouring more cash into its armed forces. If there are any diesel-electric boats the U.S. Navy must be extremely concerned about, it’s the Chinese. While China continues to develop world-class nuclear subs, it is also advancing world-class diesel-electric boats.

James F Dunnigan