It is perhaps the most unlikely kind of military base. For more than two decades, a second world war-era ship, BRP Sierra Madre, has stood deliberately grounded in the remote, shallow waters of the fiercely contested South China Sea, carrying the Philippine flag and guarding against Chinese expansion. But its future is increasingly precarious, and the ship has become a growing flashpoint in one of the world’s most disputed waters. The Sierra Madre, marooned on Second Thomas Shoal, in the Spratly Islands, since 1999, is effectively a shipwreck. Rust has ravaged its sides and holes pierce its shell. Defence experts...