(history of westeros by Unchained Maester Qyburn)
Upon declaring king of the Iron Throne from Dragonstone, From his position Stannis Baratheon realized he lacked the manpower needed to take King's Landing. With the help of the loyal lords of narrow sea and his trusted advisor Davos,,he sent messengers and promised rewards to the stormlords and secured the loyalty of some of the lords sworn to him.But majorityrefused. Then HE somehow secured his ancestral seat The Storm's End.
King Renly Baratheon rushed there with his vanguard of Twenty Five thousand men to secure it , But the young and untested king ran directly into the ambush laid by Stannis Baratheon.
King Renly Baratheon, Ser Loras Tyrell, Ser Rober Royce, Ser Emmon Cuy and nearly one and half thousand men died from crossbow bolts in the night. The rest of the men surrendered the next day to Stannis Baratheon. It was a resounding victory for the Stoic King.
King Stannis also captured lady Caitlyn Stark but instead keeping her as hostage he let her go with a ultimatum that her Son Robb Stark surrender his Crown and Re sworn his oath to House Baratheon.
However, he failed to win the allegiance of House Tyrell, who withdrew their large army from the BitterBridge and returned to Highgarden. Nevertheless, having also secured an alliance with the sellsail captain Salladhor Saan,Stannis was confident of victory. On the advice of his loyal vassal and Hand, Davos Seaworth.
With King's Landing in danger, Tyrion made preparations to withstand siege. However, most of the Lannister forces fighting a losing battle against the Young Wolf army in the Riverlands, leaving the city walls undermanned. Dowager Queen Cersei Lannister planned to level the field through the use of wildfire, a dangerous and highly destructive substance that she had made en masse that Tyrion Lannister would later use for the battle. With Robb Stark's forces poised in the west, Tywin was unwilling to leave his defensive position at Stony Sept, fearing being attacked from the rear and East where the Southern Riverlords Banded together in Herranhall with Lord Alan Winters. Their number being fifteen Thousand Strong. And half of that number's levies are sworn to the domain of Harrenhal. As if the lord had a year to prepare for this war.
Thus Tywin Lannister decided not to fight the mighty veteran lord of many wars, And approached him diplomatically. Tywin finally found his confidence to secure his alliance after the death of Renly Baratheon. And it was no secret to the realm that Alan Winters and Stannis Baratheon had no love for each other. The clash between their wife's was the top gossips for the smallfolk for five straight years.
THE war in king Robert's court ended when Lady Jenna Tyrell died in a fever and the Lord of Herranhall resigned from the court.
SO Lord Tywin Lannister gave Lord Alan a offer he could not refuse. A kingdom and a Wife.
However, when Robb launched an invasion of the Westerlands instead, drawing close to Casterly Rock, Tywin decided to move his army and new allies. At the same time, Littlefinger negotiated a fresh alliance between House Lannister and House Tyrell.