Donald Trump was seen recently performing the gesture, which has long been condemned by the Native American community, at a World Series baseball game.
Former NFL star Herschel Walker, who Trump endorsed last month in the Georgia Senate race, joined the former president and first lady at the event.
The Russian president said that Moscow would respond with "comprehensive measures" after the Pentagon test-fired a non-nuclear cruise missile.
Moscow is studying the remains of Tomahawk rockets to exploit their weaknesses.
The world's top two military powers may be on a collision course in Syria.
The U.S., Russia and China are all looking to surpass each other with military innovations such as missiles and drones that use artificial intelligence.
Trump has learned that the presidency can be an incredibly frustrating job.
Moscow called tough U.S. talk and action on the Syrian regime "flexing muscles" and "baring teeth."
The president was presented with a plan that would have included a strike on Syrian President Bashar al-Assad's palace.
Moscow had its S-400 missile system in range of the strike.
Its payload, range and navigation benefits explained.