From Quartz:
The “First Law of Petropolitics,” coined by New York Times columnist Tom Friedman… states that when oil prices are high, the leaders of petro-states can become demanding and belligerent; when they are low, they are more prone to be pussycats.
Oil prices at elevated levels explain Russian belligerence. The graph below shows Brent Crude prices adjusted by the (US) consumer price index to reflect growth in real crude prices over the last decade. To bring crude prices down to pre-2005 levels will be no small feat.
Read more at How the US might persuade the Saudis to co-conspire in unleashing an oil weapon against Putin – Quartz.