I think a lot of people are making the mistake of veiwing the Romans of Alexanders time in light of their acheivements later in history, there was nothing special about the Romans in this time, they didn't use innovative new tactics, they weren't anymore better at fighting than other peoples, their population wasn't particularly bigger than others, they weren't anymore diplomatically cunning.
What they were was a smallish regional power among others jostling for power, three years before Alexanders death they became involved in the Second Samnite war, it took 22 years for them bring it to an end and even then the Samnites were not completely defeated. If it took them that long to defeat a group of (admittedly tenacious) mountain tribes how would you expect them to do the same against a newly arisen superpower at the peak of its strength with thousands of harderned veterans lead by generals experienced in combating numerous battle tactics including those very similar to the ones the Romans used at the time.
At best they would have fought a few battles before conceeding defeat and become a client state, at worst if they were stubborn and resisted to the very end Alexander might have made a example of them and razed Rome to the ground like he did with Tyre and Thebes
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