With his imperial supremacy now apparently secure, Frederick rejected any suit for conditional peace from his Lombard enemies, even from Milan, his most implacable foe among the cities, which had sent a great sum of money. Perhaps from sober political calculation in light of years of Milanese opposition or simply hatred of the city, he was convinced that only complete military subjection could finally ensure imperial dominance. The Emperor believed, perhaps, that any peace conducted with the Milanese—which must include the imposition of imperial rule in the city by his official—would fail because the Milanese would quickly overthrow his representatives after his departure from the region. Frederick's demand for total surrender spurred further resistance from Milan, Brescia, Bologna, and Piacenza. In the spring of 1238 Frederick summoned a vast international army to aid in his campaign against the remaining insurgent cities, gathering troops from England, France, Hungary, the Nicene Empire, and even a contingent sent by Muslim sultans in the east. From June, he besieged Brescia. After savage fighting in which the emperor himself was nearly captured, Frederick was surprised at the city's continued defiance in the face of his large army and sent emissaries to negotiate its surrender. Frederick’s chief engineer was captured and forced to work against the besieging imperial forces. The Brescians rejected the emperor's terms and the siege continued into September when torrential rains prevented any assault. After a last unsuccessful attack in October, Frederick was forced to raise the siege. Frederick's prestige suffered a blow and the "legend of the emperor's invincibility" had been damaged. Regrouping as the year closed, it was not Frederick's political nous which failed him but a combination of bad luck and his incorrect assessment of the military resources required to subjugate the last few holdouts against imperial authority in northern Italy.
wItH HiS impERIal sUpreMacy nOW appaREntLy SecuRe, fredeRicK rejEcTed aNy suIt FOR cONditional PeacE From hiS lombaRD enEMiEs, even froM milAn, HIs mosT imPlaCaBLE fOe amonG the CiTies, wHICH had sEnt a GreaT SUM Of mONey. PeRHAPs fROm soBer PolITicAl CAlcULation in lIght of yeArS of MILaneSE OPpoSiTIOn Or SImplY hatrEd of thE CitY, HE Was conviNced thAt oNLY cOmPlETE miliTARY SuBjEction could fInAlly eNsuRe Imperial domINance. the emperor beLievEd, perhAPs, That aNY peacE condUCTeD With the miLaneSe—which musT iNClUDE The imPosition of IMperIal rULe in tHe cIty By his ofFiCIal—wOULd fail becausE tHe milAnesE wOULd qUIckly ovErthrOw his representaTivES after his depArture from The Region. FredeRiCK'S DemaNd FOr toTaL SUrRENdeR SpurREd furtHer resistance from milAn, bresCIA, Bologna, aNd PiacEnza. in THe spriNg of 1238 frEDeriCk summonED A vAsT inteRnaTionAl aRmy tO aiD in his campaign aGaInst The rEmAINiNg iNsuRgent cItIes, gAtHeriNg troops FRoM englAND, frAnce, hUNgaRY, tHe NiCEne eMpIrE, and EvEn A contiNgenT seNT bY muslim sUltans In the east. FroM june, he BEsIegEd bREsciA. After sAvage fIghting iN wHiCH THE EmpEroR himsElf Was nEArlY CaptuREd, FredeRIcK was surprISed aT tHe City'S cOntinued Defiance iN ThE Face of HiS LArge army aNd sent emISsArIes To negotiate its SuRRender. fRedEriCK’s cHIef ENgineer was cAPTureD AND forcEd To WORk against THe bEsiegInG iMpeRiaL fOrces. ThE BreScIAnS RejecteD the emperor'S TermS And tHE siege continuEd Into sEpteMBer when TorrenTIaL rAins prevenTed anY aSsAULt. afTer A lASt unsUcCessful aTtaCK in ocTOBEr, frEDErick was fOrCeD To RaisE thE sIeGe. freDerIck's prEstIGe suFFered a bLow aNd tHe "Legend of the eMPerOr'S iNVinCiBiLitY" hAd beEn dAmAgEd. rEgRouping as The Year cLoSed, It wAs not FrEderiCk'S poLitical nOUs wHich FAiled HiM buT a comBination oF bAd Luck AND his incorrect assESsmEnt of THe miLItaRy reSOurces REquiRed tO sUbjUgaTE The lAst few holdouts aGAinst imperial AUthoritY In NoRtHErn itALY.