powder keg
1. a keg full of gunpowder
2. a very violent or tense person
3. an area of countries with high nationalism and low tolerence for neighboring countries
1. Billy 'moonshine' Bob: pass me the powder keg, hadn't eaten this stuff in a while. 
2. She's a powder keg ready to explode
3. the balkan region was a powder keg at the turn of the 20th century