Annexation in the ancient world is not a clear as it was in the 19th and 20th century, and Rome was no exception. A few examples should show the issues with the term annexation in regards to Rome and their relationship with foreign people's. Hopefully through these examples you'll get a wider few from which you can go forward and research more yourself. Because research is the fun bit.

So firstly we will look close to home for Rome. (Rhyming was unintentional) we see extremely early from Livy that Rome was unafaird of taking hostile actions against their neighbours, a good example the rape of the Sabine women. While one may want to critise Livy by saying he is writing at the beginning if the republic thus time distorts his view do remember that coins from T. Sabinus appear to show the rape of the Sabine women in the 1st century BCE, suggesting that it was a common story in Rome.

Anyway back to my point, the history goes that the Sabine men came to Rome with force in an attempt to recapture their daughters. Before the battle of the Latins and Romans took place the women rushed out and pleaded with their husbands and their fathers to not kill each other. From this point the Latins became allies of Rome. Now after this point the Romans were the hegemonic power over the Latins and this would increase to various people's around Italy; Oscans, Samnites, Etruscans and Greeks. Some of these did have wars between them but they never truly saw themselves as annexed, more as independent cities that had various roles to Rome. We can see that at end if the 2nd century BCE and the start if the first, only really then are the allies in Italy truly pushing to be seen as Romans.

Two more clear examples of the problems of Annexation are Greece and the Attalus kingdom. Taking in chronological order Greece was not a state, as I hope you are aware, it was a collection cities states and leagues with various regions under their hegemonic control and the kingdom of Macedon projecting it's power over various Greek states. When Titus Flamininus defeating Philip V in 198 he declared freedom of the Greeks and thoroughly Hellenisitic thing to do. By doing this Rome took the place of the various Hellnenistic kingdoms that attempted to hold power in Greece since the death of Alexander by officially claiming that no one would annex the Greeks, of course Rome kept forces in Greece to firstly deal with Antiochus III and defeat him in 188 and to keep watch over Macedon. When Perseus the done of Philip did rebel and the Aetolian league with him they were defeated by Rome in 146BCE and some regions were officially occupied by Rome. However Roman control over Greece had been in place for 50 years prior to this period.

Finally, let's look at Attalus III who died in 133BCE. When Attalus died he bequeathed most of his Kingdom to Rome, with the exception of some cities. This was so important and wealthy that the Gracchi made their new holdings in Asia Minor be the bank from which they could finance their various reforms. There was indeed a rebellion from Aristonicus, who claimed to be a brother of Attalus. This rebellion was put down, but in theory the region's they fought over were not Attalid but Roman due to Attalus' will.

What these examples show is that the idea of Annexation in the ancient world is not a clear one, it drifts and varies depending on whom is writing about it, but also that Rome was effective in employing many tactics that allowed them to establish power without using force all the time. They may have the most famous military in the world, but their diplomats were just as effective.

Sources Livy - Books 1-10 for Early Rome and Italy, look at 33-38 for interactions with the Greek East.

Polybius Histories

Gruen, E Rome Enters the Greek East

Thonneman, P Attalid Asia Minor

These sources with give you a good jumping off point into more detailed view of my discussions.

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