You must have both the base game and the expansion installed. If you are using old CDs, install them normally. If you have the modern Warcraft III: Reforged client, you will need to downgrade to 1.26a, which is a different process (typically involving "old version" folders).
For many players in the West and especially in China (where internet cafes, or wangba , ran this version for years), was the definitive "vanilla" experience. It predated the modern meta of jungling ward wars and introduced heroes that are now considered classic. warcraft iii the frozen throne dota 126 install