Short answers, pulled from the story.
Maximinus Thrax became the first barracks emperor after beginning his career as an enlisted soldier. He came from outlying parts of the empire and lacked a distinguished family name or successful career as a statesman.
The Crisis of the Third Century began with the assassination of Severus Alexander in 235 AD and lasted until 284 AD. Approximately fourteen barracks emperors ruled during just thirty-three years within this timeframe.
Barracks emperors used state money to pay troops since no emperor who came to power by force could allow disaffection among soldiers. This practice damaged the economy significantly, drove up inflation across the empire, and created widespread economic hardship for ordinary citizens.
Germanic incursions intensified because frequent border commanders acting as usurpers left large defensive voids along the frontiers. These gaps allowed enemies to exploit weaknesses repeatedly while constant civil wars prevented coordinated defense strategies against external threats.
A cavalry commander named Diocletian seized power in 284 AD and ended the barracks emperor phenomenon. He instituted reforms designed to stabilize both the empire and the imperial office permanently.