You wrote: "The watchtower is a symbol of war. It must have been first created for this purpose only."
The watchtower was created not for war but for heralding of impending events, signalling the approach of something. In the heraldry of Medeival England the watchtower was a symbol of war. That has not always been the view of the watchtower.
The watchtower atop the head of Diana Artemis of Ephesus has a very different significance. It was a symbol of her connectivity to the wisdom and insight from the Great Mother and her ability to comminicate that insight to her acolytes. Later, Diana Artemis became indistinguishable from the Great Mother and to refer to one was to automatically refer to the other.
Particularly, the pagan watchtower is associated with far-sightedness, inwardly and outwardly, spiritually and prophetically. It also is believed to have properties as a defensive ward against certain forces, but not in a warlike sense. Its usage in pagan rites has always had this connotation rather than any connection to warfare.
It is specifically in this pagan context that the Watchtower Society uses the icon. Examine the cover of The Watchtower and Herald of Christ's Presence.
Isaiah 21:11-12 — The pronouncement against Du´mah: To me there is one calling out from Se´ir: “Watchman, what about the night? Watchman, what about the night?” 12 The watchman said: “The morning has to come, and also the night. If YOU people would inquire, inquire. Come again!”
It positions itself as a beacon, a watchpost for future events, a crier's perch for events that they taught occurred in 1874 and 1878. It didn't serve for war anymore than the pagan watchtower serves for war.