Would no Einstein yesterday mean no nuclear proliferation today?
Einstein's deep insight into energy and matter was sufficient to get us where we are today in terms of nuclear proliferation, but his insight wasn't necessary to our nuclear/political worries. Without him, not much would be different now.
Suppose that Einstein had not been born in 1879; indeed, suppose that he never was. Then the equation E=MC2 would not have been discovered when it was, which happened to be during WW2. WW2 was won quicker than otherwise by the allies, because they dropped A-bombs on two Japanese cities. These two bombs were dropped only to minimize allied causalities, i.e. for a political reason, not for necessary strategic reasons.
It is certain that E=MC2 would have been discovered fairly quickly, since there are several different routes in physics which point to the energy-matter equivalence so deftly captured by the equation. Maybe it would have been a year or so, maybe a decade, but it would not have taken too long, given the cumulative theoretical and technological powers of science.
Now, under the post WW2 peace-time conditions, someone probably would have suggested that one could use such a formula in making a really powerful atomic, even hydrogen bomb. As now, it would strike many as a horrible weapon to make, and many would opposed it. Still, would the most advanced industrial nations begin to make such a bomb? I think they would.
Iran is proceeding with such plans, apparently, in order to get what every nuclear armed nation gets when it engineers a bomb: instant regard from its neighbors. You have to take such a nation seriously. Iran, as have others, obtains a kind of insurance against conventional bullying by threat of military take-over. One does not waltz a military into a nation with nuclear weapons, since the cost to the invading nation is too high, indeed the cost seems to have a high probability of mutually assured destruction, a cold war axiom of political engagement used with great precision in dealing with the old Soviet Union, and an axiom still politically operant among nuclear capable nation states even today.
Since nations seek to guarantee their sovereignty against hostile military invaders, and since nuclear weapons are the most technologically efficient way of so doing, even if Einstein had not discovered the E=MC2 equation when he did, I do not believe the current problem with nuclear proliferation would be significantly different than it is today.
I think this has an interesting (hypothetical) implication. If, per horror, there would be a nuclear war, human doom would not have been sealed by Einstein, the Manhattan Project, or even by nuclear weaponry. It would really have been by entering into the process of understanding and manipulating the natural world which sealed our doom. (Consider the philosopher Aristotle's view on Hamartia) In a word, what may kill humans in the end is their propensity for Physics.
[ * ] Some argue that industrialization is what will get us, which is just a slower doom.
[ * ] Others argue that nano-technology will off us, and sooner rather than later.