Then there is the cost of disposing of obsolete craft. The terrible scale of the Russian problem is known, but the problem is not confined to them. Acquisition of submarines by countries which previously had few or none goes on. Developments initially to do with deterrence are thus closely linked with those to do with conventional conflicts. The strategy, or rather strategies, that have emerged divide the globe into two classes - those who can project power, and those who wish to frustrate that projection.
Submarines are arguably the most important single instrument for both classes.
For the supreme power projector, the US, the submarine is a valuable collector of intelligence, the safest platform for precision strikes against land targets and a vital protector of surface ships. Those who aspire to some capacity to project power, like Russia, China, Britain and France, follow, at a huge remove, down this American road. But, equally, for those who want to offer effective resistance to a descent on their shores by the forces of the US, American-led coalitions, or other big countries, the submarine is the weapon of choice. Improved propulsion systems for conventional submarines which give them more endurance and make them harder to detect, as well as better weapons and targeting techniques, mean that relatively weak countries can mount a serious threat to expeditionary forces, including the nuclear submarines that accompany them. Where such intervention depends on an assurance of low casualties, such threats might be effective.