escalate
To make a situation grow more intense or serious.
To escalate means to increase in intensity, seriousness, or scope. When something escalates, it grows bigger, stronger, or more severe, often in ways that make a situation harder to control.
A playground disagreement might start with one student making a rude comment. If the other student responds with an insult, and then pushing begins, the conflict has escalated from words to physical actions. When countries in a tense situation keep adding more troops and weapons to their borders, the crisis is escalating toward possible war.
Problems don't always escalate on their own: people can escalate situations through their choices. If you get a bad grade and respond by yelling at your teacher, you've escalated a disappointing moment into a serious disciplinary problem. People who handle conflicts well recognize when they're about to escalate a situation and choose instead to calm things down.
The opposite of escalating is de-escalating, which means reducing tension or intensity. A skilled mediator knows how to de-escalate conflicts before they spiral out of control.
Notice that escalation usually suggests something negative getting worse, though you might occasionally hear about escalating excitement or success.