Here's how it breaks down:

"Abomination of desolation" is a phrase originally from Daniel (Daniel 9:27, 11:31, 12:11), which referred to something detestable that causes desolation, typically idolatry or pagan defilement in a holy place.

Fulfillment in the First Century:

Roman armies under Cestius Gallus in 66 AD surrounded Jerusalem but withdrew. This gave believing Jews a chance to flee the city, as Jesus warned.

According to Luke 21:20, the parallel passage, Jesus said:

"But when you see Jerusalem surrounded by armies, then recognize that her desolation is near."

This clarifies that the "abomination" involved Gentile armies approaching and desecrating the holy city.

When Titus returned in 70 AD, Roman soldiers entered the Temple, burned it, and sacrificed to their standards, which they often worshiped as idols, a clear act of abomination and desecration.

Key Points:

The "abomination" was the Roman presence and idolatry in the holy place (Jerusalem and the Temple).

The "desolation" was the judgment and destruction of the city and sanctuary.

It was fulfilled within that generation, just as Jesus said in Mark 13:30:

"Truly I say to you, this generation will not pass away until all these things take place."

So, the abomination of desolation in Mark 13:14 was the encroaching Roman army, their idolatrous practices, and the profaning of the Temple, signaling believers to flee because Jerusalem's judgment was imminent.