(Reuters) -FireEye, one of the largest cybersecurity companies in the United States, said on Tuesday that it had been hacked, likely by a government, and that an arsenal of hacking tools used to test the defenses of its clients had been stolen. The hack of FireEye, a company with an array of contracts across the national security space both in the United States and its allies, is among the most significant breaches in recent memory. The FireEye breach was disclosed in a public filing with the Securities and Exchange Commission citing CEO Kevin Mandia.