The central Iraqi government based in Baghdad has conquered oil-rich and ethnically-mixed Kirkuk from its recent Kurdish rulers, who hoped to continue ruling it as part of their recently declared independent state of (Iraqi) Kurdistan, clearly consisting of three provinces, but which they also wanted to include the fourth one of Kirkuk province. This now appears not to be going to happen.
Juan Cole has made an excellent discussion of this, noting 7 reasons why this is not about Iran as many commentators in the US claim. I shall not repeat most of his arguments here but suggest people look at the link. I shall note the crucial point that what looked like it was going to be a major military conflict over Kirkuk thankfully turned out not to be is that the Kurdish Pesh Merga, who were ruling Kirkuk, actually are tied to the main opposition party in Kurdistan, the Patriotic Union Party (PUK) led by the Talabani family,whose old patriarch, once a president of all of Iraq, has just died. The Pesh Merga has simply withdrawn peacefully from Kirkuk, handing a major embarrassment to Massoud Barzani, the current president of newly independent (maybe) Kurdistan, who leads the center right Democratic Party of Kurdistan (DPK). This suggests that while the opposition nominally supported Barzani's independence referendum, they lack enthusiasm, and Barzani may end up in trouble as things are not going well with this. As I noted in a previous post, Barzani is in a tight position because he canceled an election in 2015, and Kurdistan's economy has been weak due to low oil prices.
I also add that apparently the fall of Kirkuk temporarily shuts down 350,000 barrels of oil per day production, which will add to the ongoing increase in world oil prices.