A breakthrough occurs when an offensive force has broken or penetrated an opponent's defensive line, and rapidly exploits the gap.
[1] Usually, large force is employed on a relatively small portion of the front to achieve this.
Since they were already pressured, this leads them to "snap" as well, causing a domino-like collapse of the defensive system.
The OED records "break through" used in a military sense from the trench warfare times of 1915, when the Observer used the phrase in a headline.
[2] The Online Etymology Dictionary dates the metaphoric use of "breakthrough" - meaning "abrupt solution or progress" - from the 1930s,[3] shortly after Joseph Stalin popularized the Russian equivalent (Russian: перелом, romanized: perelom) in a pep-piece on the "Great Breakthrough" published in November 1929,[4] dense with military jargon and encouraging industrialization during the Soviet Union's first Five-Year Plan.