Daenerys Targaryen set sail to Westeros at the end of Game of Thrones season 6 and landed at Dragonstone in season 7. She had three full-grown dragons, and the will to take the Seven Kingdoms. Things were going great for her, except Cersei Lannister had already begun initiating a counteroffensive. Cersei's Hand, Qyburn unveiled the Dragon-Killling Scorpion, and in Game of Thrones season 7, episode 2, "Stormborn," the duo summoned the bannermen of House Tyrell, notably the Lord of Horn Hill, Randyll Tarly, and his son, Dickon to misinform them about the supposed perils of letting the Mad King's daughter take the Iron Throne.
Truth is, Cersei couldn't care any less about her people, and she aimed at preserving the status quo. She informed the gathering about how Daenerys' "army of savages" (the Dothraki) and "mindless Unsullied soldiers" had allied with House Tyrell to rebel against the crown (the cunning Lannisters). Since Daenerys' reputation preceded her as she was the Mad King's daughter, misleading the Tyrell bannermen was a cakewalk. Jaime then led the Lannister army to the Reach and sacked Highgarden in Game of Thrones season 7, episode 4 "The Spoils of War."
Cersei Alienated The Tyrell Bannermen
Cersei realized that she was surrounded by traitors, but refused to give up without a fight. Lady Olenna Tyrell, Ellaria Sand, and Yara Greyjoy had declared for Daenerys, and in the North, Jon Snow was named King. As a start, Cersei decided to eliminate Daenerys' allies one by one. The Lannisters didn't have enough provisions to feed their army and because they were at the beginning of winter, they needed Tyrells' grain, and gold. Cersei's first order of business in Game of Thrones season 7, episode 1 "Dragonstone" was to alienate the Tyrell bannermen by telling them that siding with Olenna meant that they were siding with foreigners.
House Tyrell's bannermen broke faith and joined House Lannister to sack the Tyrell seat. It is pertinent to mention that House Tyrell was on the brink of extinction as their matriarch, Lady Olenna (the late Diana Rigg in Game of Thrones) was the last remaining Tyrell. She was in mourning at the time of the sack as Cersei nearly decimated her House by blowing up her grandson and the heir to Highgarden, Ser Loras, and her granddaughter, Queen Margaery in the Sept of Baelor. In retaliation, Olenna brokered an alliance with Daenerys of House Targaryen.
The Sack Of Highgarden
Daenerys chose not to attack King's Landing but announced that she would lay siege on its sides using the Tyrell and the Dornish armies. Her Hand, Tyrion had the Unsullied sail for the Lannister stronghold - Casterly Rock, and while Daenerys' three allies - the Dornish, the Tyrells, and the Ironborn (Yara and Theon Greyjoy) got on board, Olenna advised her to ignore her Hand and act fiercely. In Game of Thrones' "Dragonstone," Yara's rival and uncle, Euron Greyjoy arrived at King's Landing and promised to return with a gift for Queen Cersei. He kept his word, attacked Yara's fleet, and took her, Ellaria, and Tyene Sand to King's Landing.
Tyrion's clever plans had failed. By Game of Thrones season 7, episode 3, "The Queen's Justice," the good portion of the Targaryen fleet was gone, and the Unsullied were trapped at Casterly Rock without any provisions. They had no way to return to Dragonstone as Euron destroyed the fleet that ferried them to the Rock. The Lannister host marched to the Reach, with Jaime at the forefront, and Bronn, Randyll, and Dickon followed him.
Jaime gave Olenna death by poison, while she confessed to murdering his son, King Joffrey at the Purple Wedding. House Lannister profited off the Tyrell gold and was able to pay their debts to the Iron Bank. The Tarlys had the Tyrell granaries emptied and loaded the grain into wagons. They also collected the current harvest from all the farms in the Reach and Jaime saw to it that the reluctant farmers complied. As Jaime supervised this transportation, Daenerys ambushed his forces in an open field.
The Ambush
In Game of Thrones' "The Spoils of War," Randyll and Jaime aimed to get the last of the wagons over the Blackwater Rush before nightfall. They were overseeing the transportation of the grain wagons, while the gold had already made it to King's Landing. The next few moments changed everything. They prepared their men to assemble in lines as they heard battle cries from a distance. Jaime refused to abandon his infantry, hoping to hold the enemy off. But the second he and Bronn heard a roar, they realized they didn't stand a chance. Daenerys met the Lannister-Tarly army at the crossing of the upper waters of the Blackwater Rush, amid the cliffs and hills of the Goldroad. Randyll asked his men to guard the wagons and form a line, while the Lannister soldiers formed a wall. Every field tactic failed the second Daenerys said "Dracarys" and Drogon poured fire on the shield wall. The Dothraki breached the lines and fighting began.
The Battle of the Goldroad brought back into focus Robert Baratheon's words, "Only a fool would meet the Dothraki in an open field." Daenerys set the grain wagons on fire and many infantrymen tried to flee. The Dothraki cut the enemy in swift motions with their Arakhs and Jaime and Bronn looked terrified. The archers failed to get a single aim at Drogon, but Bronn managed to load the Dragon-Killling Scorpion and shoot bolts. The second bolt from the Scorpion hit Drogon in the right shoulder, and although he managed to land, he was angrier than before. He smashed the Scorpion with his tail, and as Daenerys struggled to remove the harpoon from his scales, Jaime grabbed a spear and charged at her.
The battle ending was told through Tyrion's eyes as he inspected the carnage from a distance. While Jaime thought he had a shot at Daenerys, Drogon breathed fire at him. Bronn mounted on a horse, pulled Jaime away in the nick of time and the two men fell into the Blackwater Rush with dragonfire blazing above. In the aftermath of the battle, in Game of Thrones' "Eastwatch," the Dothraki rounded up the captives, and Daenerys burned Randyll and Dickon for refusing to bend the knee. The rest immediately submitted and she departed from Goldroad knowing she offered the Tarlys a choice.