Taiwan cannot always buy what it needs. Major defense suppliers limit what they will sell to avoid diplomatic friction with China. American export control legislation restricts the transfer of sensitive technologies. Treaty allies of China will not sell Taiwan weapons at all. The result of these constraints, accumulated over decades, is that Taiwan has been forced to develop a domestic defense industrial base capable of producing the systems it cannot obtain externally — and in several categories, the systems it has developed domestically are among the most important in its order of battle.