Fourth Lateran Council c.1 (X 1.1.1): Una vero est fidelium universalis ecclesia, extra quam nullus omnino salvatur

Alanus Anglicus (ca. 1205):  Extra ecclesiam non est imperium

Those who take a vow for the defense of the Holy Land may pay others to take their place and thus redeem their vows or if they have legitimate reasons may have their departure postponed.  Infidels may not be conquered without cause.

Hostiensis (Henricus de Segusio) (ca. 1270)

Franciscus de Vitoria (1480-1536),  De Indis et de iure belli relectiones

"Again, although Sylvester (Mazzolini, also PRIERIAS) Theologian, b. at Priero, Piedmont, 1460; d. at Rome, 1523) discourses at great length on the power of the Pope, yet, under the word infideles § 7), he expressly maintains against Hostiensis that unbelievers can not be compelled by arms to recognize this lordship and can not be deprived of their property on this pretext. And Innocent maintains the same in X 3.34.8 (Quod super hiis). There is also no doubt that this was the opinion of St. Thomas too (Secunda Secundae, qu. 66, art. 8, on obj. 2)."

Two Arguments:

Quod omnes tangit ab omnibus approbari debet

Due Process

Hugo Grotius, (1583-1645), De jure belli ac pacis