In a year when Wall Street predicted oil would surpass $100 for the first time in four years, the oil market instead experienced its worst annual loss since 2015.
The oil market's sudden about-face in the fourth quarter has ended a 2½-year recovery for oil prices following the 2014-2016 downturn. Analysts expect oil prices to rebound next year, but the geopolitical risks that have weighed on the market throughout 2018 will remain a big variable in the new year.
U.S. crude settled on Monday at $45.41 a barrel, ending the year down nearly 25 percent. At around $54 a barrel, international benchmark Brent crude is down 19.5 percent in 2018. The declines mark the first annual loss and the biggest yearly drop since 2015, when both contracts fell more than 30 percent.
Just three months ago, oil was trading at nearly four-year highs. While many analysts said the rally was unwarranted and warned a pullback was in the cards, few predicted the market would sell off so sharply. From peak to trough, U.S. crude has shed nearly half its value.
With the benefit of hindsight, it's fairly easy to explain oil's plunge into a bear market.
In May, the Trump administration restored sanctions on Iran, OPEC's third-biggest producer, raising concerns about a supply squeeze in the oil market. The following month, OPEC and a group of producers led by Russia abandoned their 2016 agreement to restrict supply. At the urging of Trump and oil customers, Saudi Arabia in particular turned on the taps, adding about 1 million barrels per day to the market between June and November.