By now, you've heard a Russian plane recently flew around DC and the Trump golf course in Bedminster, New Jersey.
And while you might think it was cause to spool up the THAAD and drop that plane in its tracks, believe it or not, they were allowed to by a 25-year-old treaty based on an idea that was nearly four decades old at the time.
The Treaty on Open Skies was first proposed by Dwight D. Eisenhower in 1955. Cold War paranoia meant it went nowhere for 37 years. After the coup that proved the end of the Soviet Union, the treaty was eventually signed by President George H. W. Bush and ratified in 1992. But it didn't enter into force until 2002.
The treaty allows the U.S. and Russia — as well as a number of other NATO and former Warsaw Pact countries — to make surveillance flights over each other's territory.
According to a letter to the Senate included with the treaty, this is to "promote openness and transparency of military forces and activities." Certain planes are equipped with four types of sensors, optical panoramic and framing cameras, video cameras with real-time display, infra-red line-scanning devices, and sideways-looking synthetic aperture radar. These suites are used to monitor military forces, and are certified by observers.
Which aircraft is used can vary. The United States uses the OC-135B Open Skies aircraft for this mission. Canada uses a modified C-130. Russia has a version of the Tu-154 airliner. The United Kingdom has used a mix of planes.
The exact number of flights a country may have varies, but the United States and Russia each get 42 such flights a year.
They can fly any sort of flight plan – as long as they give 72 hours notice prior to the arrival. The flight must be completed in 96 hours from the time that the plane arrives. The plane on the Open Skies mission also must embark observers from the host nation on board.
So that's why a lot of people in the Virginia, Maryland, and DC area got a good look at a Russian Tu-154 — and may still see more if Putin wants another closer look.