Please answer a question about the following article about Roman Republic:

Beginning in the summer of 54 BC, a wave of political corruption and violence swept Rome. This chaos reached a climax in January of 52 BC, when Clodius was murdered in a gang war by Milo. On 1 January 49 BC, an agent of Caesar presented an ultimatum to the senate. The ultimatum was rejected, and the senate then passed a resolution which declared that if Caesar did not lay down his arms by July of that year, he would be considered an enemy of the Republic. Meanwhile, the senators adopted Pompey as their new champion against Caesar. On 7 January of 49 BC, the senate passed a senatus consultum ultimum, which vested Pompey with dictatorial powers. Pompey's army, however, was composed largely of untested conscripts. On 10 January, Caesar crossed the Rubicon with his veteran army (in violation of Roman laws) and marched towards Rome. Caesar's rapid advance forced Pompey, the consuls and the senate to abandon Rome for Greece. Caesar entered the city unopposed.

Ultimately what forced the senate to abandon Rome and flee to Greece?
Caesar's rapid advance