All the Muslim's enemies band together in a final attempt to defeat them.
In the year 5 AH, the two banished Jewish tribes ally with the Meccans to mobilizing the masses for the ultimate battle against the Muslims, in what would later be known as the Battle of the Azhab, aka Battle of Khandaq.
The Quraysh had been humbled by their constant failure to wipe out the Muslims, and now that Muslims were blocking their trade routs the Quraysh were feeling the pain in their wallets. Abu Sufyan, receptive to the invitation by the Jewish tribes, rallies the Meccans and other Pagan tribes to raise the largest army the Arab peninsula had ever seen so far and they begin marching against Medina.
The Muslims in Mecca learn of this, and with Salman Al-Farsi's advice, begin digging a tranch along the city's border.
The banished Jews reach out to the one Jewish tribe still remaining in Mecca and convince them to also break their truce with the Muslims (now that a huge army is coming towards the city). The tribe openly reveals that they'll break their covenant, and the Muslims realize they have to guard against enemies both outside and inside Medina.

Lecture notes available at http://www.why-quran.org/?p=574.

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