|Although this article is based on canonical information, the actual name of this subject is pure conjecture.
- Olenna Tyrell: "And now the rains weep o'er our halls. Did we fight well?"
- Jaime Lannister: "As well as could be expected."
- Olenna Tyrell: "It was never our forte. Golden roses indeed."
- — Olenna and Jaime after the fall.[src]
The Sack of Highgarden is an engagement during Daenerys Targaryen's war for Westeros. With the backing of House Tarly, Jaime leads a Lannister host to the Reach, the region ruled by the Tyrells, and sacks their seat of power, Highgarden.
During the War of the Five Kings, House Tyrell backed Renly Baratheon until his death. Afterwards, the Tyrells secretly brokered an alliance with House Lannister and helped them defeat Stannis at the Battle of the Blackwater. In exchange, Joffrey Baratheon wed Margaery Tyrell. Joffrey was eventually poisoned at his own wedding, and his younger brother Tommen ascended to the Iron Throne and wed Margaery.
Cersei believed the Tyrells to be their rivals and were threatening the Lannisters' hold over the Iron Throne. After the assassination of Tywin, Cersei plotted against the Tyrells, elevating the religiously fanatic Sparrows to power in the Faith of the Seven and using them to incarcerate Loras and Margaery. However, her plan backfired, and the High Sparrow also had Cersei incarcerated and humiliated. Eventually backed into a corner by the Sparrows when the High Sparrow's influence grew over Tommen, Cersei had the the Great Sept of Baelor destroyed. Within seconds, the Faith Militant was obliterated - as were Loras, Margaery, and Mace Tyrell, ending the main line of House Tyrell.
Olenna Tyrell, who had been the true power of the Reach, had fled King's Landing earlier. When she received word of the deaths of her son and grandchildren, she traveled to Dorne, where she entered into an alliance with House Targaryen, agreeing to back Daenerys's claim to the Iron Throne.
With the Lannister armies heavily depleted from the War of the Five Kings due to fighting Stannis and Robb Stark, Cersei planned to have Highgarden sacked and plundered of its grain and wealth to feed the Lannister armies. Furthermore, the crown owed a tremendous debt to the Iron Bank of Braavos. After summoning the lords of the Reach to King's Landing, Cersei attempted to persuade them to turn against the Tyrells; Jaime later met privately with Randyll Tarly, one of the most prominent bannermen of the Tyrells, and convinced him to join the Lannisters. Jaime also promised Randyll the position of Warden of the South and lordship of the Reach if he helped the Lannisters defeat their enemies, which Randyll reluctantly accepted.
Tyrion, acting as the Hand of the Queen to Daenerys, believes that the Lannister armies were still in the Westerlands defending Casterly Rock. He sends the Unsullied army, led by Grey Worm, to take the Rock. However, Jaime in fact marches a Lannister host to the Reach with the backing of Randyll Tarly, and together sack Highgarden, crushing the House Tyrell garrison.
After Highgarden falls to the Lannister-Tarly host, Jaime meets privately with Olenna, who puts together the tactics Jaime had used to march to Highgarden and defeat her forces. Jaime remarks he learned from his defeat by Robb at the Battle of the Whispering Wood. He then pours poison into a cup of wine and offers it to her as opposed to taking her back to King's Landing to face the "justice" of his sister. After drinking the poisoned wine, Olenna confesses to murdering his and Cersei's son Joffrey, imploring Jaime to tell Cersei that it was she who committed the murder.
The fall of Highgarden and House Tyrell also marks the loss of Daenerys's last Westerosi ally, which will seriously hinder her efforts to retake the Iron Throne; even more so if the other lords of the Reach or Dorne don't support her cause. It also seriously hinders her political stance with Jon, and the situation may force her to concede to his demands if she were to ask for his help in retaking the Iron Throne.
The remaining gold and food left after the battle is taken under control of Jaime and is transported to King's Landing so that Cersei can feed the Lannister armies and pay the remaining debts of the Iron Bank, in order to take out a new loan.
In retaliation, Daenerys realizes she needs to be more upfront with the fact that she has three dragons, so she attacks the Lannister-Tarly forces with Drogon at the Battle of the Goldroad. The majority of the grain seized from Highgarden is destroyed in the battle, although the gold safely reached King's Landing beforehand.
In the books
In the fourth novel, while the Lannisters are still allied with the Tyrells, Euron launches a military campaign against the Reach. The Lannisters are not actively involved in that conflict; their only "contribution" is Cersei's refusal to send the Redwyne fleet back from Dragonstone in order to drive the ironborn away. The ironborn manage to conquer the Shield Islands, the Arbor and several of its neighboring islands, even threatening Oldtown; perhaps Euron plans to take Highgarden as well, but currently he focuses on the imminent naval battle against the Redwyne fleet.
Daenerys Targaryen's war for Westeros has not begun yet. Instead, her presumed-dead nephew Aegon Targaryen (son of Rhaegar and Elia), who has the strongest claim to the throne, and his guardian Jon Connington land at Wesetros with nearly half of the Golden Company, and begin their campaign of conquest. The Lannisters and the Tyrells, while engaged in a soft power struggle, remain allies. Cersei's scheme against Margaery has nearly broken that alliance, but Kevan takes steps to appease the Tyrells: Mace Tyrell is appointed as Hand of the King for King Tommen, and two of his bannermen - Randyll Tarly and Paxter Redwyne - are granted seats at the Small Council as Master of Laws and Master of Ships respectively.
The "Friends in the Reach" theory, however, states that Randyll may be among the Reach lords that will turn on House Tyrell.