fall in

break down, literally or metaphorically

• “The wall collapsed”
• “The business collapsed”
• “The dam broke”
• “The roof collapsed”
• “The wall gave in”
• “The roof finally gave under the weight of the ice”

to take one''s place in a military formation or line

• “Troops fall in!”

become part of; become a member of a group or organization

• “He joined the Communist Party as a young man”