Here are some extracts from History of the Conquest of Spain by the Arab-Moors, Volume 1 by Henry Coppée, published in 1881. You often find an unusual amount of truth in these old books written before the era of political correctness set in.
I have already spoken of one important constituent in the combinations of Ilyan [traitor general of the Goths, also known as Julian], upon which, as it has asserted itself in all periods of Spanish history, it seems proper to dwell a little more of more at length: I refer to the Jews. Nothing is sadder, while nothing is more unique, more entirely sui generis, in the history of the world than the separate and peculiar existence, of this injured and everywhere persecuted people, who have been -- especially since the Christian era, which their blind and cruel act of unbelief inaugurated -- despised, trodden down, hunted, exiled, tortured, and killed. And yet quite as striking is the moral power which they have wielded over their persecutors. While the Christians were slothful in business, slack in industry, and wasted what they had, the Jew gained and hoarded; "accommodated" the spendthrifts with usurious loans, and appealed for precedent to Jacob's stratagem, --
The attitude of the Jews.

"This was the way to thrive, and he was blessed; And thrift is blessing, if men steal it not."

It was only thus that they could revenge themselves on their oppressors. They were thus brought into relations, which must now be considered, at once with the Christians in Spain and with the Arab-Moors in Africa, and out of a combination of these relations
they emerge to view as a potent element in the Arabian conquest and after dominion in the Peninsula.

Of their first coming thither there is no certain record. We may believe that when the fleets of Solomon made their voyages to Tharshish, -- "for the king had at sea a navy of Tharshish with the navy of Hiram: once in three years came the navy of Tharshish, bringing gold and silver, ivory, and apes and peacocks," -- a few of the adventurous Israelites remained in the Peninsula, and formed a nucleus for others who, when Judea was overrun by hostile armies from time to time, left their country, and wandering along the northern coast of Africa, and hearing tidings of their brethren in Spain, joined them there. We know that when the Romans conquered the Peninsula they found a considerable number of Israelites domiciled there. When, after the terrible siege of Jerusalem, by Vespasian and Titus, "one stone was not left upon another" of the city of their love and pride, crowds of exiles wandered westward to swell these numbers. Spain was a quiet Roman province, and there by their industry, frugality, and skill in business, the Jews made themselves useful members of society.

There were many who remained in northern Africa, and who were in constant and cordial intercourse with their brethren in Spain. There they might have fondly hoped that their wanderings were at an end: not so; fierce persecutions were in store for them there as elsewhere.

When Heraclius became emperor of the East, and determined to punish the Jews in his dominion, his ambassadors made a treaty with the Gothic monarch, "Sisebuto", one of the articles of which required that all recusant Jews should be driven out of Spain. This article was also embodied in the Visigothic Code. By its terms one year was given to them in which to decide whether they would confess Christ and be baptized, or be shaved and scourged, their property confiscated, and themselves forced to leave the country. In such a fearful contingency the majority became hypocrites. Ninety thousand are said to have submitted to baptism; but the enforced Christian rite was but a mask for the circumcision which was still secretly active, and they were thus transformed from quiet and orderly subjects into concealed and intriguing foes. But even those who patiently submitted were not, as we have seen, secure from humiliation and new indignity;they were despised for their apostasy, which was but a new proof to their tormentors of their sordid character.

The fourth council of Toledo, held in the year 633, indeed revoked the former decree requiring them to be baptized; but this apparent clemency was neutralized by the cruel requirements that the children of those who had accepted Christianity should be taken from their parents to be more fully educated in the Christian faith, and that those Jews who had married Christian women should either embrace the religion of their wives, or be separated from them. It was further decreed by the council that, in a judicial trial, no Jew could give evidence against a Christian.

This placed the Jews at a fearful disadvantage. The rigor increased. The sixth Toledan council,in 638, was more outspoken, and not much more cruel when it enacted "that Judaism would not be tolerated in the realm;" the eighth council prescribed new rigors against them. These enactments produced in part the desired effect; large numbers banished themselves, taking refuge in Africa from Christian baptism and persecution; there too they were ready to join any respectable conspiracy aginst the government and the people who had so constantly oppressed them. For this scarcely concealed purpose at least they received full credit; and when Egica ascended the throne in the year 687, they were under special surveillance, as it was averred that the Jews in Spain and Africa had entered into a special agreement to aid in the destruction of the Gothic monarchy. Thus, while to hate and persecute the Jews was considered an undoubted part of Christian duty in the abstract, their reported conspiracy added fuel to the flame. We may therefore fancy the astonishment of the Gothic nation and of the Christian world when Witiza, to serve his own ends as it was believed, removed the anathema and disabilities, and restored them partially at least to a condition of security and ease.

This apparent clemency disgusted his subjects, not so much because he set aside ecclesiastical canons and secular laws as because he dared to run counter to the universal and unrelenting prejudice which ignorantly based itself upon the claims of Him who had forgiven His enemies upon the cross.

Let us turn for a moment to see how they were regarded by the Arab-Moors. Their early relations to the creed of Mohammed have already been presented. We have seen that they had been powerful in Arabia before the advent of the prophet. Princes had embraced the Law of Moses, and the efforts of Mohammed were strenuous to convert them. When he found this a very difficult task, by the general claims of Islam, he asked for, and received, special revelations denouncing them: in numerous passages of the Koran their unbelief is rebuked and their fate declared. Notwithstanding this, they do not seem to have been regarded with the same disfavor as other unbelievers: they were among the first people allowed to compound for tribute; and, if still despised, were permitted to live in peace.

But now, in northwestern Africa, they had risen in importance. What rendered them dangerous to the Gothic Christians gave them new value in the eyes of the Arab-Moors, who were making ready to invade the Peninsula. These disaffected and confederate Jews formed a band of intelligent and useful auxiliaries in the scheme of the Moslem conquest. The martial sounds of the Moslem hosts made pleasant music in their ears. National allegiance they had none. They had the warrant of history that the change of masters would ameliorate their condition: they would aid and serve the kindest. Mohammedan Spain would be better than Christian Spain, because it would be more tolerant. For the Christian Messiah and for the prophet of Islam they had equal disregard; and thus the readers of later Spanish history will find that, in troublous times, they often, like soldiers of fortune, changed sides, and not unfrequently held the balance of power through the influence of their unity and their wealth. As a single illustration: When the Moslems began to persecute them for their money they turned to the Christians and brought to the throne Alfonso VI., of Castile and Leon, in the year 1085.

As we read of their checkered fortunes, we are struck with the fact that the important part they have played has been purposely ignored or belittled by both parties to the struggle; but it is not difficult to discover the truth, in spite of the reluctant mention or intentional silence of both Spanish and Arabian historians, -- the former prompted by religious rancor, and the latter by a pride of conquest which would not share the glory with such humble agents. It seems certain that, in concerting his plans for the conquest, Musa had early taken the Jews into his counsels: he received valuable information and gained important statistics from them which they had learned in the way of trade.

It is also wetted that after Count Ilyan had been in communication with certain disaffected Goths in Spain, he also sought the aid of the Jews, as an important element in carrying out his purpose. They gladly listened to both commanders, and probably supplied money, which their sagacity assured them was as safe and profitable an investment as in those turbulent times they could make.

We may now return for a moment to Roderik and the impending invasion as it actually took shape before his eyes. His newly acquired power seemed well established; his kingdom was a fair show. He could still defy "malice domestic and "foreign levy." He had indeed his suspicions of Gothic disaffection, and every day brought stronger corroboration of the presumptuous purpose of the Arab-Moors; he knew that the children of Israel were secretly leagued against him, but thus far he had no doubt of his ability to withstand all these united enemies, and place the Gothic power upon an eminence of authority and glory which it had never yet attained. If he had entertained misgivings as to the fidelity of Ilyan, he seems to have dismissed them.

...The final reduction of Cordova left Mugheyth free to employ his troops, in conjunction with those of Tarik, in such a manner as his chief should direct. It was now the end of August, 711. He placed, as had been done elsewhere, the local authority in the hands of the Jews of the town, the only people he could trust, and fortified his control by taking hostages from among the principal men. He made the palace his head-quarters, thus inaugurating its future grandeur and power. Retaining around him a sufficient garrison, he then spread the remainder of his troops through the Comarca, awaiting the orders of Tarik. From this time Cordova remained in Moorish hands, continually growing in power and splendor, until, with the Moorish dominion, it began to decline in the early years of the eleventh century.

...The advance of Tarik had been necessarily slow and cautious; but the case seemed hopeless. The fame of his victories had struck terror into all hearts. The chief nobles and warriors upon whom. the people might have relied in their extremity had fallen on the field. Those who had fled into Toledo only thought of further flight. And every day brought new testimony to the valor and number of the Moslems, and the ubiquity of their light and fleet horsemen. There were in the city no munitions of war: the paralyzed inhabitants had collected no store of provisions. There could be no hope of succor from without, and while, in the last resort, the Christians were going in sad procession to invoke the assistance of St. Leocadia in their great exigency, the vanguard of Tarik appeared before the town. If the Christians were in despair, the Jews, who had dissembled their joy, scarcely waited for his summons to counsel an immediate surrender.

(Gayangos further quotes, "that the Jews opened the gates of the city to the Moslems.")

...Meanwhile, the second division under Zeyd Ibn Kassed, had proceeded, without delay, to conquer Malaga and overrun its Comarca. He met with little resistance, and was soon able to send or take a strong detachment to Gharnatta, the Medina or capital of the district of Al-Birah. they found no opposition; and here, also, they found a large number of Jews, well advised of their coming and their purpose, ready to welcome the invader, and glad to find in the Berber ranks many of their brethren, who, although converted to Islam, retained the instincts of consanguineous blood. Into their hands, Zeyd gave the government of Gharnattah, assured of their energetic co-operation in the Moslem schemes. The number of Jews in that town and the power reposed in their hands, caused the place to be called, in all its earlier history, Gharnatta- al-Yahood, Granada of the Jews.
Source: History of the Conquest of Spain by the Arab-Moors, Volume 1 by Henry Coppée, 1881

Islam versus Europe: Jewish Collaboration with Muslims during the invasion of Spain, Part II