At the end of the 4th century, Ausonius enumerated Aquileia as the ninth among the great cities of the world, placing Rome, Constantinople, Carthage, Antioch, Alexandria, Trier, Mediolanum, and Capua before it.
However, such prominence made it a target and Alaric and the Visigoths besieged it in 401, during which time some of its residents fled to the nearby lagoons.
Alaric again attacked it in 408.
Attila attacked the city in 452.
During this invasion, on July 18, Attila and his Huns so utterly destroyed the city that it was afterwards hard to recognize its original site.
The fall of Aquileia was the first of Attila's incursions into Roman territory; followed by cities like Mediolanum and Ticinum.
The Roman inhabitants, together with those of smaller towns in the neighborhood, fled en masse to the lagoons, and so laid the foundations of the cities of Venice and nearby Grado.

Yet Aquileia would rise again, though much diminished, and continue to exist until the Lombards invaded in 568; the Lombards destroyed it a second time in 590.
Meanwhile, the patriarch fled to the island town of Grado, which was under the protection of the Byzantines.
When the patriarch residing in Grado reconciled with Rome in 606, those continuing in the Schism of the Three Chapters, rejecting the Second Council of Constantinople, elected a patriarch at Aquileia.
Thus, the diocese was essentially divided into two parts, with the mainland patriarchate of Aquileia under the protection of the Lombards, and the insular patriarchate of Aquileia seated in Grado being protected by the exarchate of Ravenna and later the Doges of Venice, with the collusion of the Lombards.
The line of the patriarchs elected in Aquileia would continue in schism until 699CE.
However, although they kept the title of patriarch of Aquileia, they moved their residence first to Cormons and later to Cividale.

The Lombard Dukes of Friuli ruled Aquileia and the surrounding mainland territory from Cividale.
In 774, Charlemagne conquered the Lombard duchy and made it into a Frankish one with Eric of Friuli as duke.
In 787, Charlemagne named the priest and master of grammar at the Palace School Paulinus the new patriarch of Aquileia.
The patriarchate, despite being divided with a northern portion assigned to the pastoral care of the newly created Archbishopric of Salzburg, would remain one of the largest dioceses.
 Although Paulinus resided mainly at Cividale, his successor Maxentius considered rebuilding Aquileia.
However, the project never came to fruition.

While Maxentius was patriarch, the pope approved the Synod of Mantua, which affirmed the precedence of the mainland patriarch of Aquileia over the patriarch of Grado.
However, material conditions were soon to worsen for Aquileia.
The ruins of Aquileia were continually pillaged for building material.
And with the collapse of the Carolingians in the 10th century, the inhabitants would suffer under the raids of the Magyars.

By the 11th century, the patriarch of Aquileia had grown strong enough to assert temporal sovereignty over Friuli and Aquileia.
The Holy Roman Emperor gave the region to the patriarch as a feudal possession.
However, the patriarch's temporal authority was constantly disputed and assailed by the territorial nobility.

In 1027 and 1044 Patriarch Poppo of Aquileia, who rebuilt the cathedral of Aquileia, entered and sacked neighboring Grado, and, though the Pope reconfirmed the Patriarch of the latter in his dignities, the town never fully recovered, though it continued to be the seat of the Patriarchate until its formal transference to Venice in 1450.

In the 14th century the Patriarchal State reached its largest extension, stretching from the Piave river to the Julian Alps and northern Istria.
The seat of the Patriarchate of Aquileia had been transferred to Udine in 1238, but returned to Aquiliea in 1420 when Venice annexed the territory of Udine.

In 1445, the defeated patriarch Ludovico Trevisan acquiesced in the loss of his ancient temporal estate in return for an annual salary of 5,000 ducats allowed him from the V
At the end of the 4th century, Ausonius enumerated Aquileia as the ninth among the great cities of the world, placing Rome, Constantinople, Carthage, Antioch, Alexandria, Trier, Mediolanum, and Capua before it. However, such prominence made it a target and Alaric and the Visigoths besieged it in 401, during which time some of its residents fled to the nearby lagoons. Alaric again attacked it in 408. Attila attacked the city in 452. During this invasion, on July 18, Attila and his Huns so utterly destroyed the city that it was afterwards hard to recognize its original site. The fall of Aquileia was the first of Attila's incursions into Roman territory; followed by cities like Mediolanum and Ticinum. The Roman inhabitants, together with those of smaller towns in the neighborhood, fled en masse to the lagoons, and so laid the foundations of the cities of Venice and nearby Grado.
Yet Aquileia would rise again, though much diminished, and continue to exist until the Lombards invaded in 568; the Lombards destroyed it a second time in 590. Meanwhile, the patriarch fled to the island town of Grado, which was under the protection of the Byzantines. When the patriarch residing in Grado reconciled with Rome in 606, those continuing in the Schism of the Three Chapters, rejecting the Second Council of Constantinople, elected a patriarch at Aquileia. Thus, the diocese was essentially divided into two parts, with the mainland patriarchate of Aquileia under the protection of the Lombards, and the insular patriarchate of Aquileia seated in Grado being protected by the exarchate of Ravenna and later the Doges of Venice, with the collusion of the Lombards. The line of the patriarchs elected in Aquileia would continue in schism until 699CE. However, although they kept the title of patriarch of Aquileia, they moved their residence first to Cormons and later to Cividale.
The Lombard Dukes of Friuli ruled Aquileia and the surrounding mainland territory from Cividale. In 774, Charlemagne conquered the Lombard duchy and made it into a Frankish one with Eric of Friuli as duke. In 787, Charlemagne named the priest and master of grammar at the Palace School Paulinus the new patriarch of Aquileia. The patriarchate, despite being divided with a northern portion assigned to the pastoral care of the newly created Archbishopric of Salzburg, would remain one of the largest dioceses.  Although Paulinus resided mainly at Cividale, his successor Maxentius considered rebuilding Aquileia. However, the project never came to fruition.
While Maxentius was patriarch, the pope approved the Synod of Mantua, which affirmed the precedence of the mainland patriarch of Aquileia over the patriarch of Grado. However, material conditions were soon to worsen for Aquileia. The ruins of Aquileia were continually pillaged for building material. And with the collapse of the Carolingians in the 10th century, the inhabitants would suffer under the raids of the Magyars.
By the 11th century, the patriarch of Aquileia had grown strong enough to assert temporal sovereignty over Friuli and Aquileia. The Holy Roman Emperor gave the region to the patriarch as a feudal possession. However, the patriarch's temporal authority was constantly disputed and assailed by the territorial nobility.
In 1027 and 1044 Patriarch Poppo of Aquileia, who rebuilt the cathedral of Aquileia, entered and sacked neighboring Grado, and, though the Pope reconfirmed the Patriarch of the latter in his dignities, the town never fully recovered, though it continued to be the seat of the Patriarchate until its formal transference to Venice in 1450.
In the 14th century the Patriarchal State reached its largest extension, stretching from the Piave river to the Julian Alps and northern Istria. The seat of the Patriarchate of Aquileia had been transferred to Udine in 1238, but returned to Aquiliea in 1420 when Venice annexed the territory of Udine.
In 1445, the defeated patriarch Ludovico Trevisan acquiesced in the loss of his ancient temporal estate in return for an annual salary of 5,000 ducats allowed him from the V
Aquileia