Hollow Crowns and Deadly Thrones:
Renly’s appeal to military supremacy has a certain pragmatic sense given that he has an army of 100,000, which is a staggering size for a medieval army.
In the Middle Ages, logistical shortcomings meant that armies of this size were not practical, with an average size for medieval armies of 10-20,000 men. As late as the 16th century, armies tended to top out at the 40,000 mark. The “military revolution” of the early 17th century was where things really started to change, with armies in the 100,000 and above range becoming common for major military powers. (see “The Military Revolution in Early Europe” by David Parrott, in in History Today Volume: 42 Issue: 12)
When you have an army bigger than all your other rivals combined. arguments that military strength should trump everything definitely favors his argument in the short-term. When Renly offers to Catelyn to count his camp fires, as:
"You will still be counting when dawn breaks in the east … I’m told your son crossed the neck with twenty thousand swords at his back … Now that the lords of the Trident are with him, perhaps he commands forty thousand … I have twice that number here … and this is only a part of my strength."
Catelyn II, A Clash of Kings
there is no denying the crude strength of his argument that his opponents should bend the knee because he possesses a predominance (if not a hegemony) of military force, lest they be destroyed outright. In many ways, it’s the same argument Aegon made to his peers before the Conquest.
In the long-term, however, it’s an extremely dangerous political theory for the stability of the Westerosi monarchy. Renly has the most troops at that moment, but there’s no way to be sure that Renly or his descendant, or his descendant’s descendant will have the same numerical advantage in the future. If his argument is accepted as binding precedent, Renly will forever have to remain on his guard lest someone out there strike while he is unaware since it’s now accepted that a strongman can legitimately overthrow a sitting king. Even if he succeeded in holding the Iron Throne for the duration of his natural life, the odds are good that his death will set off a new civil war as each of the Great Houses assesses the new balance of power.