In 539 BC, Belshazzar and the gold head of Babylon (Iraq) fell and Cyrus the Great ruled the silver arms and chest of Media-Persia (Iran). See Daniel 2. Darius (or Gobyras) was the Persian general that conquered and governed the Babylonian territory under King Cyrus. He assigned three leaders to oversee a hundred twenty rulers. Despite being a leader in the Babylonian empire, Daniel was chosen first leader because of his excellent spirit. God was still using Daniel to protect the Jews in captivity.

It pleased Darius to set over the kingdom an hundred and twenty princes, which should be over the whole kingdom; And over these three presidents; of whom Daniel was first: that the princes might give accounts unto them, and the king should have no damage. Then Daniel was preferred above the presidents and princes, because an excellent spirit was in him; and the king thought to set him over the whole realm (Daniel 6:1-2, 3)

The 122 leaders under Daniel were jealous that he was chosen first leader. They investigated his work but could not find any fault. However, they found that he was faithful to his God and not the gods of Persia or or the influence of man.

Then the presidents and princes sought to find occasion against Daniel concerning the kingdom; but they could find no occasion nor fault; forasmuch as he was faithful, neither was there any error or fault found in him. Then said these men, We shall not find any occasion against this Daniel, except we find it against him concerning the law of his God (Daniel 6:4-5).

Daniel's adversaries propose a law. For thirty days, no leadership in the kingdom shall pray to God or man except for the king. A person found breaking the law is thrown into a den of lions. This helps the Babylonians unify with the new Persian leadership. The leaders bypass Daniel, appeal to the Darius' pride while hiding their true intentions. According to the customs of the Meads and Persians, once a law is signed, it cannot be altered.

All the presidents of the kingdom, governors, princes, counsellors, and captains have consulted together to establish a royal statute, and to make a firm decree, that whosoever shall ask a petition of any God or man for thirty days, save of you, O king, he shall be cast into the den of lions. Now, O king, establish the decree, and sign the writing, that it be not changed, according to the law of the Medes and Persians, which cannot be altered (Daniel 6:7, 8)

Daniel knew the law was signed. As part of Daniel's faith, he consistently prayed three times a day. He had a choice to obey man or obey God who has authority over man. He prayed and gave thanks in his house with the window open as he faced Jerusalem. He trusted that the God of Israel who has been faithful sixty-six years in captivity would protect him from his adversaries.

Now when Daniel knew that the writing was signed, he went into his house; and his windows being open in his chamber toward Jerusalem, he kneeled upon his knees three times a day, and prayed, and gave thanks before his God, as he did aforetime (Daniel 6:10).

Daniel's adversaries had other plans. They caught Daniel praying and immediately went to Darius. You signed a law that says no one would pray to any God or man for thirty day or be cast into a den of lions. Daniel of Judah does not respect you or the law but prays three times a day. When the king heard their accusation, he was upset because he was deceived into passing the law and was concerned with delivering Daniel. The rulers reminded the king that the law cannot be changed.

Then they answered and said before the king, That Daniel, which is of the children of the captivity of Judah, regards you not, O king, nor the decree that you have signed, but makes his petition three times a day. Then the king, when he heard these words, was sore displeased with himself, and set his heart on Daniel to deliver him: and he labored till the going down of the sun to deliver him (Daniel 6:13, 14).

Darius commanded that Daniel be put in the lions' den. He told Daniel that his God who he serves will deliver him. While Daniel was in the lions' den, the king was in the palace having a long night without sleep and food. Early in the morning, the king went to the den of lions. Daniel, servant of the living God, is your God able to deliver you from the lions? My God has sent angles to shut the mouths of lions. God has found me incident before Him and before the king. Darius commanded that Daniel be taken up out of the lions' den.

Then the king commanded, and they brought Daniel, and cast him into the den of lions. Now the king spoke and said unto Daniel, Thy God whom you serve continually, he will deliver you.
Then the king went to his palace, and passed the night fasting: and his sleep went from him. Then the king arose very early in the morning, and went in haste to the den of lions. And when he came to the den, he cried with a mournful voice unto Daniel: and the king spoke and said to Daniel, O Daniel, servant of the living God, is your God, whom you serve continually, able to deliver thee from the lions?
My God hath sent his angel, and hath shut the lions' mouths, that they have not hurt me: forasmuch as before him innocence was found in me; and also before thee, O king, have I done no hurt (Daniel 6:16, 18-19, 20, 22).

Darius commanded that Daniel's accusers be thrown into the den of lions. Unlike Daniel's experience, his adversaries were not innocent and the lions made quick work of them. Darius made a degree throughout his territory that men fear the God of Daniel. Then Daniel prospered in the reign of Cyrus the Great and Darius of Persia. 

The king commanded, and they brought those men which had accused Daniel, and they cast them into the den of lions, … and the lions had the mastery of them, and brake all their bones in pieces [before] they came to the bottom of the den.
I make a decree, That in every dominion of my kingdom men tremble and fear before the God of Daniel: for he is the living God, and steadfast for ever, and his kingdom that which shall not be destroyed, and his dominion shall be even unto the end (Daniel 6:24, 25-26)

In the New Testament, Hebrews 11 is the Faith Hall of Fame. Daniel is mentioned as his faith "stopped the mouths of lions". During the Tribulation, the 144,000 witnesses will be sealed and protected but many Tribulation Saints will receive salvation but die for their faith.

What shall I more say? for the time would fail me to tell of Gideon, Barak, Samson and Jephthah; of David, Samuel and of the prophets: Who through faith subdued kingdoms, wrought righteousness, obtained promises, stopped the mouths of lions (Hebrews 11:32-33).

References: 21st Century Revelation: World Wars, Iraq Wars & End War Chapter 8 - Prophecy Fulfilled, Chapter 13 - Minor Feasts, Major Events, Daniel's Dream, Rapture, Tribulation, Millennium, Daniel