One thing can imperil another when it threatens to be harmful. For example, an approaching storm with hurricane-force winds might imperil the geraniums you just planted in your front yard.
A city's budget cuts might imperil a school's ability to hire new teachers and buy supplies. The lack of money, in other words, is a threat to schools. Likewise, a factory's refusal to reduce its greenhouse gas emissions will imperil the air quality nearby — and ultimately, imperil the health of the whole planet. The word imperil comes from in and peril, meaning "danger."