The following units can be upgraded to Destroyers:
Destroyers have a small chance of intercepting enemy aircraft and also are very good at spotting and destroying enemy Submarines (which are invisible to most other units). They are often stacked with Carriers and Transport units.
Destroyers are small escort ships that serve a variety of useful functions. These light, fast vessels were used extensively in World War II to provide air and sea cover for carrier groups. One of the primary functions of the destroyer is to locate and destroy enemy submarines. Using their sonar equipment to locate the subs, and their batteries of depth charges and torpedoes to destroy them, the destroyer forces of World War II were the key to the success of many crucial naval operations. Modern destroyers are still used in much the same role, as escorts for larger vessels and as submarine hunters. In addition to five-inch guns, the arsenal of today's destroyer includes anti-submarine helicopters, anti-ship cruise missiles, and the Phalanx system for defense against anti-ship missiles.