He is chiefly remembered for his advocacy of a decisive policy against the Ottoman Empire, and his command of Venetian forces as Captain General of the Sea during the lead-up to and the first stages of the First Ottoman–Venetian War.

Soon after he arrived at Corfu, however, the Republic reversed course, fearing a lasting military commitment against the Ottoman Turks, and decided to simply establish a Venetian protectorate over the Tocco domains, rather than annex them.

Cappello's actions during the war are not well known; at the start of the conflict, when the new Captain General of the Sea, Alvise Loredan, was still in Venice, he conducted raids against the Neapolitan shores, but in August he was ordered to keep watch over the entry of the Straits of Otranto and prohibit any Aragonese vessels from operating in the area.

With a resumption of the long-standing conflict with the Duchy of Milan being imminent, in May Cappello was sent as capitano (military commander) of Brescia, and was active in restoring its fortifications and preparing its defence.

[1] The news of the Fall of Constantinople caused a widespread rebellion to break out in the Despotate of the Morea among the local Albanian population against the weak rule of the brothers Thomas and Demetrios Palaiologos.

Venetian suspicions of Genoese naval activity in the area also led to suggestions to try to secure various port towns—Patras, Vostitsa, Glarentsa, and Corinth—for Venice, to prevent them from coming under the control of "another maritime power", but this proposal failed to pass in the Senate.

[1][3] Cappello arrived in Venice and began visiting the warring parties in an effort to reconcile them, but in the event, the revolt was ended through the intervention of the Ottoman warlord Turahan Bey in October 1454, who forced the Albanians to submit, and restored the feeble authority of the two Despots.

[1] Their mission was delicate, as, aside from conveying the usual messages of congratulation, the envoys were charged with representing the Republic's positions on the location of a general congress of the Christian powers for a coordinated attack on the Ottomans, that the new Pope intended to convene.

Following the conquest of the Despotate of the Morea in 1460, Ottoman pressure on the Venetian possessions of Modon and Coron increased, with raids being launched against them by Turkish forces in February 1461.

Accordingly he was limited to ensuring the safety of the Venetian possessions, and that the Ottoman fleet did not leave the Dardanelles, although he was free to engage the light fusta vessels that the Turks used to conduct piracy in the Aegean.

[1] Even when the Ottomans occupied Lesbos in September 1462, Cappello, heeding his instructions, refused to answer the repeated calls for aid and observed events from Chios.

[1] Cappello did nevertheless take care to prepare for the oncoming conflict by overseeing the reinforcement of fortifications in Coron, Modon, Nauplia, Lepanto, and Negroponte, until his tenure expired in November.

[1] Cappello initially belonged to the more moderate faction, which advocated a policy of "armed neutrality", strengthening the fleet and Venetian fortifications overseas, but without entering into commitments with other Christian powers in the Balkans against the Ottomans.

By the time the Doge's fleet arrived at Ancona to meet with the Pope, however, Pius II was ill from the plague, and died three days later, on 15 August, ending any designs for a crusade.

[8] Cappello favoured a decisive campaign in the Morea, and in the early months of the war, he seemed to get his wish; the Venetian forces retook Argos and refortified the Isthmus of Corinth, restoring the Hexamilion wall and equipping it with many cannons.

They then proceeded to besiege the fortress of the Acrocorinth, which controlled the northwestern Morea, but after a clash that killed the commander-in-chief, Bertoldo d'Este, on 20 October, the Venetians were then forced to lift the siege and retreat to the Hexamilion.

After a while, demoralized and riddled with dysentery, the Venetians abandoned that position too and withdrew to Nauplia, allowing Ottoman reinforcements under Mahmud Pasha Angelović to enter the Morea unimpeded.

[9] In February, Cappello argued for the appointment of Sigismondo Pandolfo Malatesta as the new commander-in-chief,[1] but his tenure, which lasted through 1465, was ineffective, mostly due to the limited resources and manpower made available to him.

Two attacks on Mytilene in 1464 failed, and the Captain General of the Sea, Jacopo Loredan, spent the remainder of his time in ultimately fruitless demonstrations of force before the Dardanelles,[11] and with a diplomatic confrontation, that nearly led to war, with the Knights Hospitaller of Rhodes.

Cappello nevertheless came too late: a few days before his arrival, Turahanoğlu Ömer Bey with 12,000 horsemen attacked the Venetian forces at Patras and routed them, killing Barbarigo.