Smallpox has an r0 of 3: in a completely naive population, each person with the disease will infect 3 other people. If 80% of the population is immune, the disease should eventually die out.

Polio has an r0 of about 6, and needs about 80-85% of the population to be immune to prevent it's spread. [There's a whole debate about the use of IPV vs OPV in the struggle to eliminate polio, which is interesting if you want to delve into it.]

Measles is among the most contagious diseases we've ever encountered; it has an r0 of about 14. To get measles to die out, you need a minimum 95% immunity rate. 3% of the people who get vaccinated for measles will never develop sufficient immune response to the vaccine and will still be able to catch and spread the disease. That leaves a razor-thin 2% leeway before herd immunity starts to fail. And there's way more than 2% of the US population in the '57-'67 and '67-'89 cohorts who haven't gotten a booster to make herd immunity problematic.

Do anti-vax communities provide vectors for the introduction of measles? Absolutely; the have been many outbreaks among the extreme Orthodox Jewish communities in New York and Northern New Jersey City that exact reason; and the current big outbreaks originated with old school Mennonites in the Southwest.

Does the decrease in vaccination rates enable measles to spread much more rapidly and easily than before? Absolutely!

But the reason measles is spreading in the US has multiple causes: its incredible contagiousness, the decrease in herd immunity, a lack of funding for public health campaigns; the rise of cellphones and scammers/spammers; the absolute gutting of the public health infrastructure; and other elements that I'm not currently thinking of. Reducing the issue to "antivaxxers bad" minimizes other contributing factors.

I'm heavily pro-vaccine and I'm not defending antivaxxers; I'm just seeing this as a more complex issue than other people seem to.