Russian SU-24 bombers flew 30 feet over the US destroyer Daniel Cook in what the Pentagon referred to as a "simulated attack."  The incident in the Baltic occurred in international waters.

Military Times reports on other aggressive actions by Russian warplanes against U.S. ships:

Shortly after leaving the Polish port of Gdynia, near Gdansk, on Monday, the Donald Cook at was sea in international waters conducting flight operations with a Polish helicopter, part of routine joint training exercises with the NATO ally.

During those flight operations, a Russian Sukhoi Su-24 combat aircraft appeared and conducted about 20 overflights, coming within 1,000 yards of the ship at an altitude of about 100 feet, the defense official said. In response, the commander of the Donald Cook suspended flight operations.

On Tuesday, the Donald Cook was underway in the Baltic Sea when a Russian helicopter — a Ka-27 Helix — made seven overflights and appeared to be taking photographs of the U.S. Navy ship, the defense official said.

Shortly after the helicopter left the area, an Su-24 began making “very low” overflights with a “simulated attack profile,” the defense official said. The aircraft made a total of 11 passes.

The ship's commander repeatedly tried to make radio contact with the Russian aircraft but received no response, the defense official said.

After a formal investigation, the incident may prompt the U.S. government to formally lodge a complaint — or "demarche” — with Moscow, the defense official said.

Ooooh – the dreaded "strongly worded letter."

I don't know what the rules of engagement are in these situations, but perhaps if U.S. ships would light up the Russian planes by locking on their weapons, that might discourage these incidents. 

The "simulated attack" may violate a 1973 treaty with the Soviet Union that bars such maneuvers against our ships.  Of course, treaties never stopped Putin before.  His violation of the Intermediate Range Nuclear Forces Treaty has elicited little more than a formal protest from the U.S.

Here's a video of the encounter:

Putin has been probing for Western weakness over the last few years.  The annexation of Crimea, the fighting in eastern Ukraine, flexing his muscles in Syria – Putin is playing sophisticated game of chess, and frankly, the Western response to his moves has been less than adequate.

What would the response to an incident like this be from any of the presidential candidates?  Probably not very different from President Obama's.  Putin knows the U.S. won't risk war over trifles like this.  But over time, these little incidents begin to add up and paint an overall picture of U.S. weakness against Russian strength.