Two words that, to this day, still ring throughout history.
The first word, μολών, is the aorist active participle (masculine, nominative, singular) of the Greek verb βλώσκω "blōskō," meaning "having come." Λάβε is the aorist active imperative (second person singular) of the verb λαμβάνω "lambanō," translated "take [them]."
The two words function together in a grammatical structure not present in English called the circumstantial participle. Where English would put two main verbs in two independent clauses joined by a conjunction: "come and take", a strategy sometimes called paratactic, ancient Greek, which is far richer in participles, subordinates one to the other, a strategy called hypotactic: "having come, take." The first action is turned into an adjective. In this structure, the participle gives some circumstance attendant on the main verb: thecoming.
In regard to aspect, the aorist participle is used to signify completed action, called the perfective aspect. Moreover, the action must be completed before the time of the main verb. The difference in meaning is subtle but significant: the English speaker is inviting his enemy to begin a process with two distinct acts or parts—coming and taking; the Greek speaker is telling his enemy that only after the act of coming is completed will he be able to take. In addition there is a subtle implication: in English "come and take it" implies that the enemy might not win the struggle—the outcome is uncertain; in Greek, the implication is that the outcome is certain—"after you have come here and defeated me, then it will be yours to take." For comparison, these elements happen to be present in the previously-noted English phrase, "over my dead body", or the similar phrase "I'll give up my gun when you pry it from my cold, dead hands."
Μολὼν λάβε was reportedly the defiant response of King Leonidas I of Sparta to Xerxes I of Persia at the onset of the Battle of Thermopylae (480 BC). Xerxes, whose forces vastly outnumbered the Spartans and their allies, offered tospare the lives of Leonidas and his few thousand warriors if they would only surrender and lay down their weapons.
Instead, the Spartans held Thermopylae for three days and, although they were ultimately annihilated, they inflicted serious damage upon the Persian army, and most importantly delayed its progress to Athens, providing sufficient time for the city's evacuation to Salamis Island. Though a clear defeat, Thermopylae served as a moral victory and inspired the troops at the Battle of Salamis and the Battle of Plataea.
The source for this quotation is Plutarch, Apophthegmata Laconica, 225c.11. This work may or may not be by Plutarch (ca. 46 - 127) himself, but it is included among the Moralia, a collection of works attributed to him but outside the collection of his most famous works, the Parallel Lives.
 Modern usage
Molon labe has been repeated by many later generals and politicians in order to express an army's or nation's determination to not surrender. The motto ΜΟΛΩΝ ΛΑΒΕ is on the emblem of the Greek First Army Corps, and is also the motto of United States Special Operations Command Central (SOCCENT).
In the Anglosphere, both the original Greek phrase and its English translation are often heard from pro-gun activists as a defence of the right to keep and bear arms. It began to appear on pro-RKBA web sites in the late 1990s and early 2000s. In the wake of firearm seizures during Hurricane Katrina and the subsequent defiance of Federal court orders by the New Orleans government to return seized weapons, the phrase has again gained popularity among Second Amendment supporters.
Molon labe has been recently used in the 2007 feature film 300 in which Leonidas speaks this famous line in English in response to "Spartans! Lay down your weapons!" as "Persians! Come and get them!" In the 1999 comic book of the same name, upon which the film is based, the phrase becomes "Come and get it", with no exchange concerning the laying down of arms. In the earlier 1962 film The 300 Spartans Leonidas says the phrase both in Greek and English to the Persian general Hydarnes. The same exchange contains Dienekes' remark about "fighting in the shade" (as Persian arrows would "blot out the sun"), assigned to Leonidas.
If, for one minute, anyone in this country thinks that millions of Americans will just bow out of protecting ourselves from a persecutor that comes from within our own borders, you've been sadly, sadly mistaken. If you still think this, and you plan to make laws prohibiting firearms outright, or even try to ban certain ones, say, Automatic weapons, you will be exacted a price so high it will make the 'bailout' seem like small potatoes.
This isn't a threat, it's just a pulse I've gotten from everyone I know who owns a gun. There will be no peace where gun owners are concerned regarding gun bans, be they automatic or otherwise. They, We, will not let them go. No apologies, no surrender, no defeat. Period. Zilch. End of conversation.
Long live the fighters; may the light shine upon you in dark times.