wikipedia.org
Rituals For War
When it came to declaring war there were a number of rituals that were traditionally performed. A specialized body of priests known as fetials had a number of different rituals that they needed to perform in order for war to be declared by the Senate. The final part of the ritual was to throw a spear into enemy territory. This worked well enough for a time but eventually the territory of Rome grew to be far too big for this to be practical. Rome often did not even share borders with their enemy or the enemy was much too far away from the fetials for them to throw the spear.
Most were ready to do away with this part of the tradition but the fetials were too superstitious to let it go. They found a rather ingenious way to keep up the tradition without actually having to travel long distances or through enemy territory. They found a spot of land near the temple of Bellona (the goddess of war) and declared that spot of land to be non-Roman. When the Romans decided they wanted to go to war against King Pyrrhus of Epirus in 280 BC they needed to throw a spear into his territory. So the Roman Legion captured one of King Pyrrhus’ soldiers and forced him to buy the piece of land near the Bellona temple. Then the fetials were able to throw the spear onto this land that now belonged to the enemy they wished to fight.