Friendship is Casus Belli
In the Court of Princess Celestia
Princess Celestia, by dint of her millennia at the reins of Equestria as its reigning diarch, managed not to roll her eyes, smiling serenely as her nephew trotted up, pulling a scroll from his saddlebags. "What's this, Blueblood?"
"As ever, Auntie Celestia, I come bearing and symbolizing the will of the nobility of Equestria." He grinned, his teeth shining the white of the city on a mountain.
She returned it indulgently, and opened the scroll, skimming it. "Hmm…what does Parliament want now… ‘We the united hm-hmm-hmm of Equestria… whereas, whereas, whereas… therefore… declare war on Twilight Sparkle?!’ What makes you think you can do this, my little pony? The Parliament was never granted the power to declare war."
"Princess…Auntie, at least have the courtesy to read the document. This is no act of Parliament. And, even if the Kingdom of Unicornia was degraded to a principality by my forebear Princess Platinum in Bullion's memory, and you took up Unicornia's historic magical duties and were therefore invested as Princess Celestia, it was never stripped of its sovereignty."
"It requires unanimous consent of its lords to declare war, in absence of the same from its royalty."
He frowned. He didn't like frowning, as his regal smile helped secure his position as Most Eligible Bachelor of Equestria. He liked being ignored even less, though, especially when he was right. "We have it."
Celestia read the document more thoroughly, including the bit with her nephew's John Hancock with a prominent number three prepended, on down in a line of numbered signatures to 173. A quick count showed he was correct. "All present, indeed…but most of these smaller fiefs surrendered to me ages ago, and some not of Unicornia. By what right may they declare war?"
"The Celestia Charta reserves to the subordinate fiefs the right to declare war in times of serious discord."
"The… a pair of wings is not a matter of Discord!" she spluttered. She flicked her tail and, with a small spell, sent a sprinkling of salt over her shoulder and behind her. Discord sneezed, withdrawing the pair of wings he was planning on attaching to her rump, and shriveled up to bother other ponies more appreciative of his love.
"On the contrary, Auntie, the Clarifying Edicts of…well, it was 1330EE* , that specified that matters of succession in which any noble was affected granted them this right," he rejoined. (*15 BNM by the new calendar's reckoning)
"But Princess Twilight is not in the lines of succession for any of these territories," she said. Her serene smile was dimming; usually her mastery of subtleties of law, along with a planted aide in his entourage, allowed her to shoot down his more outlandish propositions. It seemed he…nay, they had covered their bases this time.
"But we are in the succession of Equestria, and she has been declared a Princess thereof, equal to Aunt Luna, and is likewise to replace you," contended nominally-Prince Blueblood. "So each of us share a line of succession with her, as necessary. Furthermore, we had an alumnus of your School for Gifted Unicorns return and cast this fixed spell, allegedly by Starswirl the Bearded that allegedly elevated her to ceraptery†, which you took as justification to crown her Princess. However, our caster did not spontaneously grow wings nor transcend our existence, despite showing deft faculty in shuffling around the cutie marks of her volunteer subjects, and in many other ways sharing traits with our new so-called Princess; even if her mane was styled a little differently. It casts doubt on the… merit of Twilight's ascension."
Princess Celestia grew unsettled as he laid his case, brick by brick, resisting her rebuttals. She decided to pursue the practical angle, asking, "And with what army might you wage this war? Our Royal Guard is not yours to command; they are sworn to Us directly." Surely her nephew did not intend to sully his hooves and horn himself waging war on her protégé?
Smirking, Blueblood produced another roll of parchment from his panniers. "See this second scroll of signatories, Auntie."
"All two hundred two of the knighthoods residing in Canterlot, and three from Ponyville? They can't declare war, either," she nigh-pled, sweating. The guards flanking the throne shifted nervously; they had not seen her so discomfited… well, in their lifetimes, at least.
Blueblood consulted the main scroll and read aloud, as law was not his talent. "Whereas the so-called Symbolic Investiture Proclamation of 75 ANM inducts Knighthood of Equestria into succession, Dame and Sir everyponyone," he trailed off. "And messenger pterripi are off collecting more signatures from beyond the city, as well."
Princess Celestia read his war proclamation once more, now with care. A line leapt out at her from the nobles signatory: ‘108. Velvet, Marquess Twilight, abstains.’ She pounced on the discrepancy. "Wait, House Twilight did not assent, but abstained, as did, I see, Fancy. Is not unanimity a universal assent?"
At this, Pants the Duke of Fance cleared his throat from further back in the audience hall. and trotted up when beckoned. "I assent if necessary," he ventured, "but the Charta requires unanimous consent, not a unanimous yea vote."
The Princess was taken aback. She had always counted on his support in matters like these. "I thought better of you, Duke Pants. You were ever a voice of reason among my little ponies. Why do you promote this disharmony? "
He simply replied, "Favors," and, though unhappy with her little spypony, she understood; he could probably have counted it as one to every other house listed, a staggering quantity of political capital amassed in one fell signature. Moreover, Fance always had an independent streak.
"Also, Auntie, Velvet thoughtfully provided a letter explaining that she does not oppose me in this." He presented a scroll in his magic, sealed by wax imprinted with the Twilight house insignia. She broke it and perused the missive. Somehow, she did not let her expression break, though its wording amused her. "So you're telling me you declare war-"
"Yes, Auntie, we would declare war because Twilight Sparkle cut in line."