So, we have pretty strong reasons to date the Gospel of Mark to somewhere between 66AD and 70AD- a time during which the Romans were doing what the Romans did, and waging war in Judea.
We have references to ongoing war in Judea, most notably:
Mark 13:5-8 and 13:14-19.
We also have references to persecutions of Christians in:
Mark 8:34-38 and Mark 13:9-13
Both the war in Judea and Nero's persecution of Christians are decidedly historically 'discrete' (they fit within quite a narrow time frame) which gives us reasonable accuracy with dating it compared to the 'broader sweeps' we have with the other Gospels.
As a direct response to your contention:
My contention with that view is why would the Christians write a polemic account about the romans? They would have been killed!
is two-fold:
A) It's hardly unimaginable that a group of religiously motivated people would write polemic attacks on a foreign occupying power despite there being some threat of reprisal. People act in spite of danger all the time.
B) Roman authority wasn't as direct as you suggest. Judea's exact legal status at the time was rather unclear. It was no longer a legitimate client kingdom, but nor was it a full roman province. Rather, it had an Augustan Procurator- which was an equestrian role normally associated with a legionary province (that is, one in which public order required the presence of Roman military might) that was in the process of becoming 'fully Roman'.
As such, it's not as if the Romans were a police state in the style of 1984. They were closer to an occupying power somewhat akin the US in Afghanistan. Around major metropolitan centers, Roman presence would have been very strong, but in more rural areas (such as Galilee) there would likely have been significant anti-Roman sentiment.
Add to that the Roman military action in Judea at the time, and it seems perfectly reasonable that Mark would have had few qualms about writing something that doesn't necessarily paint the Romans in the best light.
In short, I would conclude that the Gospel of Mark is probably the Gospel whose dating we're most secure on because it has some link to wider geo-political events whose dating we're much more sure on.
(The stuff about Romans is all based of a very wide range of sources- but I would suggest Natalie Lewis' Roman Civilization as one of the most thorough books on the subject, if you were interested)