|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 really our forte. Golden roses, indeed!"
- — Olenna and Jaime after the fall.[src]
The Sack of Highgarden is an engagement during Daenerys Targaryen's invasion of Westeros. With the backing of House Tarly, bannermen of House Tyrell, Ser Jaime Lannister leads a Lannister host to the Reach, the region ruled by the Tyrells, and sacks their seat of power, Highgarden.
The Tyrells of Highgarden and Lannisters of Casterly Rock were previously allies in the War of the Five Kings, though Cersei Lannister saw them as rivals, in particular Margaery Tyrell, to influence over the Iron Throne, which was then still controlled by the Baratheon dynasty. She later extinguished the main line of House Tyrell when she destroyed the Great Sept of Baelor in King's Landing. The matriarch of the main line and the true power of the Tyrells for decades, Olenna Tyrell took control of House Tyrell and formed an alliance with Daenerys Targaryen, seeking vengeance for the death of her family.
Ser Jaime sacked Highgarden to plunder it of its wealth and grain to feed the heavily depleted Lannister armies and pay back the crown's debts to the Iron Bank of Braavos, which had been called in by Tycho Nestoris. Jaime promised Lord Randyll Tarly, who believed the Dothraki to be a threat to Westeros, the position of Warden of the South, which was traditionally held by the head of House Tyrell.
During the War of the Five Kings, House Tyrell of Highgarden, the lords of the Reach, backed Renly Baratheon until his death at the hands of Melisandre, an advisor to Stannis Baratheon. Afterwards, the Tyrells secretly brokered an alliance with House Lannister and pledged fealty to Joffrey Baratheon. Ser Loras Tyrell, heir to Highgarden, rode with Lord Tywin Lannister and helped defeat Stannis at the Battle of the Blackwater. In exchange for coming to Joffrey and the Lannisters' aid, Joffrey agreed to wed Margaery Tyrell, Loras's sister who was previously wed to Renly, who had in fact been Loras's lover. Joffrey was eventually poisoned at his own wedding, and his younger brother Tommen Baratheon, also a bastard born of incest between Cersei and Jaime Lannister, ascended to the Iron Throne and was wed to Margaery. Though Tyrion Lannister and Sansa Stark had been accused of poisoning Joffrey, it had in fact been Petyr Baelish and Olenna Tyrell, the matriarch of House Tyrell.
Cersei Lannister believed the Tyrells to be their rivals and were threatening the Lannisters' hold over the Iron Throne. After the assassination of Tywin Lannister by Tyrion, 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 Baratheon, Cersei had the Great Sept of Baelor destroyed with wildfire on the day of her trial. Within seconds, the Faith Militant was obliterated - as were Loras, Margaery, and Mace Tyrell, ending the main line of House Tyrell.
Mace's mother, 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 Targaryen's claim to the Iron Throne, which Cersei Lannister now sat upon after Tommen Baratheon committed suicide when he heard of Margaery's death. The Targaryens had ruled the Seven Kingdoms for nearly three centuries until they were overthrown in Robert's Rebellion about two decades before the War of the Five Kings.
With the Lannister armies heavily depleted from the War of the Five Kings due to fighting Stannis Baratheon and Robb Stark and winter having officially been declared by the Maesters, Cersei planned to have Highgarden sacked and plundered of its grain and wealth to feed the Lannister armies. Furthermore, the gold mines of Casterly Rock had run dry under the rule of her father despite his strengthening of the Lannister armies, and the crown owed a tremendous debt to the Iron Bank of Braavos which was being called in by Cersei's rule. After summoning the lords of the Reach to King's Landing, Cersei attempted to persuade them to turn against the Tyrells; Ser Jaime Lannister later met privately with Randyll Tarly, one of the most prominent bannermen of the Tyrells and a childhood friend of Olenna, and convinced him to join the Lannisters, appealing to his fear of the brutal Dothraki, who had been brought over to Westerosi shores for the first time in history. Ser 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. Later, Cersei was met by Tycho Nestoris of the Iron Bank, who reminded her of the debts that the Lannister family owed the Iron Bank, to which Cersei replied that they would be paid in full soon.
Tyrion Lannister, who had been named Hand of the Queen to Daenerys Targaryen, believed that the Lannister armies were still in the Westerlands defending Casterly Rock. As a result, he sent the elite Unsullied army, led by Grey Worm, to take the Rock. However, Jaime Lannister had in fact marched a Lannister host to the Reach with the backing of Randyll Tarly, and together sacked Highgarden, crushing the Tyrell garrison.
AftermathAfter Highgarden falls to the Lannister-Tarly host, Ser Jaime meets privately with Olenna Tyrell, 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 Stark at the Battle of the Whispering Wood during the War of the Five Kings. 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 Baratheon, at the Purple Wedding, 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 Targaryen'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 Snow, 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 of Braavos, in order to take out a new loan.
In retaliation, Daenerys realises 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
Daenerys Targaryen's invasion of Westeros is anticipated but has yet to begin, and 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 will be among the Reach lords that will turn on House Tyrell.