Mutually Assured Destruction
For millions of years, humans resolved problems by poking each other with sticks. It worked well enough—you poked the person until they stopped poking you, and then the fight was over.
Then we got the idea to put fire on the end of the sticks, which worked better. Of course, there was the occasional huge wildfire that wiped out a dozen farms, but overall it worked pretty well.
Then we got the idea to put fire at the other end of the sticks...
Humans eventually decided that they would stop throwing missiles around and would stick to things that were a little bit less effective.
Of course, you're in a fantasy setting (I presume), so things are a bit different...
The Spring has around for as long as anyone in the kingdom can remember. In fact, even the records of the Blind Folk tell of their explorers hearing the Spring when they emerged from the caves. It did not take long for the race of man to learn to draw from it.
When a skilled wizard is tuned into the Spring, they can access tremendous power. It was within the power of a team of wizards to lift a vast city into the sky. But it was within the power of an individual to topple it.
Quickly, the land that is now the kingdom became consumed by ceaseless war. Strong dark wizards used local rulers as puppets to wage war with each other, and the rest of mankind suffered.
After thousands of years, humanity finally emerged from the darkness with only six hundred people left. These people founded a new kingdom from the ashes of the old one, and that kingdom stands to this day.
Today, children are taught about the Spring in their history classes, and the lucky few who can hear it from birth might unwittingly draw from it from time to time. But nobody wants to deliberately use it for fear of what happened centuries ago.
When the Kingdom was founded, millions of Spring-linked playing cards were manufactured and distributed all over it. These playing cards became the only common use of the Spring. The cards could be lethal, sure, but they had a very limited impact: a single full-length game would take hours and could only kill up to one person at a time.
Soon people realized that, instead of armies wielding swords, they could wield cards. Card combat was clean—it caused no civilian casualties, spread no dangerous disease, and required no long treks through the wilderness to engage the enemy. By now, no state in the Kingdom has a "blood army," and nobody wants to try and create one for fear of an escalation.