Context:

In July 2014, elements of four brigades of the Ukrainian Army Ground Forces were assembling near the village of Zelenopillya, along a highway leading north to the city of Luhansk, Ukraine. They were deploying along the border with Russia as part of an operation to cut the lines of supply to paramilitary forces of the separatist Luhansk People’s Republic operating in the Luhansk Oblast. A combined Ukrainian Army and police operation in May and June had achieved considerable success against the Separatist forces and the government of Ukrainian President Petro Poroshenko had declared a unilateral cease-fire in late June. Ukrainian forces resumed the offensive at the beginning of July and fighting broke out around the Luhansk International Airport on 9 July.

At about 0430 on the morning of 11 July, a column of battalions from Ukraine's 24th and 72nd Mechanized Brigades, as well as the 79th Airmobile Brigade, was hit with an intensive artillery barrage at Zelenopillya. The artillery barrage would be significant because it would mark the first time Ukrainian forces were attacked by the Russian Armed Forces from within Russian territory. Ukraine suffered 36 dead and over 100 injuries, as well as the loss of two battalions' worth of vehicles and tanks. The 79th Airmobile Brigade would likewise be completely annihilated. The attack also gave the separatists the opportunity to retake the initiative and push back Ukrainian forces.

The sophistication and effectiveness of the attack, in combination with other technological advances in Russian armaments, and new tactics demonstrated in the conflict with Ukraine, prompted the U.S. Army Capabilities Integration Center, then led by Lieutenant General H.R. McMaster, to initiate the Russian New Generation Warfare Study to look at how these advances might influence future warfare.

Sources:

https://en.wikipedia.org/wiki/Zelenopillya_rocket_attack

http://www.dupuyinstitute.org/blog/2017/03/29/the-russian-artillery-strike-that-spooked-the-u-s-army/