I was reading about Mount Pelée and the disaster relief that followed the eruption. I was surprised to read the large number of countries that would soon be rivals in WWI that sent aid to France. This got me wondering if this behavior is something we see throughout history.
Are there any examples of disaster relief to rival nations in pre-modern times?
Absolutely. In 464 the Peloponnese was hit by a massive earthquake which nailed Lacedaemonia particularly hard. The Athenians, led by the pro-Spartan Cimon, sent a contingent of 4000 men to help their then-allies, and in particular to suppress the massive helot revolt that had broken out in the confusion. Thucydides claims that the Spartans feared that the Athenians would join the helots rather than suppressing them and asked the army to return to Attica. The Athenians were naturally insulted and immediately broke off their alliance with the Lacedaemonians (and Cimon, the big supporter of democratic conservatism and the alliance with the Spartans, began to lose favor), which was never renewed. Ironically, because they were turned away the Athenians ended up giving the helots support through Messenia, using their fleet and marines to evacuate thousands of Messenian helots, who were armed at the public expense and settled as a garrison at the important Athenian naval base at Naupactus, which could effectively cut Corinth off from the rest of the Peloponnese. These troops became hugely important to Athenian success in the First Peloponnesian War, which broke out in 460 due to the now seriously strained relations between Sparta and Athens, and the early part of the Peloponnesian War
Not quite a natural disaster, but when the Kai province ruled by the Takeda clan were cut off from supplies of salt by neighboring states, the rival Uesugi Kenshin of Echigo sent them salt, so there is a Japanese term "sending salt to your enemies" to refer to showing compassion for your enemy. This would be in the 16th century Japan.