One success of Russian foreign policy in recent years has been a remarkable improvement in relations with Turkey, historically a regional rival with which it fought multiple wars. So why Russia would put those gains at risk to start flying combat aircraft into Turkish airspace takes a bit of explaining.

It was only on Sept. 23, just days before Russia launched its air campaign in Syria, that President Recep Tayyip Erdogan was in Moscow to open a new mosque with his friend Vladimir Putin. Erdogan lauded the way trade between the two countries had risen to $31 billion last year — in 2002, the year Erdogan's Justice and Development Party won power, that figure was $5 billion. The Turkish leader set a target of $100 billion by 2020.

In April, contractors broke ground for a nuclear plant on Turkey's coast, a $20 billion Russian contract. Last year, the two countries agreed to build a pipeline taking up to 63 billion cubic meters of Russian gas to Europe each year. Several times over the last two years, Erdogan has talked with Putin about joining the Shanghai Cooperation Organization, the security body Russia formed with China and four Central Asian states.