A Unitarian Universalist Church minister and U.S. Army chaplain who served in Afghanistan has submitted his resignation in protest over the military's use of drone warfare and decision to place nuclear warheads on air-launched cruise missiles.

In an April 12 letter to President Barack Obama, Rev. Christopher John Antal of New York said the White House "continues to claim the right to kill anyone, anywhere on earth, at any time, for secret reasons, based on secret evidence, in a secret process, undertaken by unidentified officials. I refuse to support this policy of unaccountable killing."

Antal's position on drone killings isn't new -- he condemned it during a sermon to troops and civilian contractors in 2012 in Kandahar, where he was a battalion chaplain.

That led to his unrequested departure from Afghanistan, a do-not-promote recommendation and a separation from the Army that was overturned only after a congressional inquiry, he told Military.com on Wednesday.

Antal is a minister for the Unitarian Universalist Congregation at Rock Tavern, New York, a chaplain at the Department of Veterans Affairs Medical Center in Philadelphia, and a founder of the Hudson Valley, New York, chapter of Veterans for Peace.

Antal's letter of resignation to Obama wasreprinted on the UUC Rock Tavern website last month, as well as on the Veterans for Peace site. In it, he also offered among his reasons his refusal to support a nuclear weapons policy that costs billions to maintain and threatens human existence.

He was further troubled by Obama's decision to support a nuclear air-launched cruise missile, he told Military.com. Before writing and sending off the letter, he said he and his congregation at Rock Tavern wrote letters to the White House urging the administration to change its drone and nuclear policies.

But what finally forced his hand to resign was a the February report on Obama's targeted drone killing policy from The Stimson Center, a nonpartisan think tank, which gave the administration largely failing grades in meeting earlier recommendations to reform the program and make it more transparent.

"That upset me. That was a big part of it," he said. "I didn't see any progress."

In addition to spelling out his opposition to the drone policy and nuclear weapons, Antal wrote that he could not support "preventive war, permanent military supremacy, and global power projection" and "imperial overstretch."

Capt. Eric Connor, deputy chief of media relations for the Army Reserve Command at Fort Bragg, North Carolina, confirmed the service is processing Antal's resignation but said its current status is pending. Until it is final, Antal remains a chaplain with the 354th Transportation Battalion at Fort Totten, New York, with his assigned higher headquarters the 377th Theater Support Command, New Orleans, Connor said.

Connor said Antal, like all soldiers, enjoys freedom of speech under the Constitution.

"We encourage our soldiers to have a conscience and let their conscience be their guide," he said. "But sometimes their conscience leads to other things, and in this case we're letting them know if your conscience says the military is not for you, we totally understand."

'A Veterans Day Confession'

Antal's opposition to drone warfare and the White House's program of targeting killings was known to some Army officials at least since 2012, when mentioned it at Kandahar Air Base, Afghanistan.

Antal -- then a lieutenant -- spoke out against the program to troops, civilian employees and contractors who attended his Unitarian service at Kandahar, where he said he was assigned to a signals battalion supporting the 3rd Infantry Division.

He recapped the sermon -- which he called "a Veterans Day Confession for America" -- last month during a symposium on drone warfare at the University of Nevada Las Vegas Law School.

"I confessed to broken promise to Afghan interpreters, I confessed to extrajudicial assassinations and death by remote control," he said inthe speech recorded and posted online. "I confessed to war made easy without due process, and I confessed to a national closet bursting with skeletons with innocent dead, killed by our government in the name of America."

He said he preached the sermon because he believes that "public confession" is one of the best ways to examine the collective conscience, purify intentions and to relieve the"moral injury" that service members can experience when they witness something or act in ways that conflict with their ethical or moral beliefs.

He also said he considered the sermon to be an act of "speaking truth to power," which he said Army Regulations supported until just over a year ago, when it changed some of the wording ofAR-165-1 (Army Chaplains Corps Activities). Section 3, paragraph 3 now reads: "Chaplains, in performing their duties, are expected to speak with candor as an advocate to confront and support resolution to challenges and issues of the command."

At the time he spoke in Kandahar, he said, the same regulation stated that "Chaplains, in performing their duties, are expected to speak with a prophetic voice and must confront the issues of religious accommodation, the obstruction of free exercise of religion, and moral turpitude in conflict with the Army values."

Military.com could not find the earlier version on the Army site but it is posted on the website of the Military Association of Atheists and Freethinkers.

An Army spokeswoman told Military.com on Friday that the change was made to be more inclusive of the diverse religions/denominations represented by Army chaplains.

"The reference to a prophetic voice has a Judeo-Christian background and history," Tatjana Christian said. "The concept of a prophet may not be common to other religions/denominations such as Buddhist or Hindu. The function of speaking 'truth to power' by Army chaplains still exists in our regulation."

Antal said he took seriously that he was expected to speak "with a prophetic voice" in confronting "moral turpitude in conflict with the Army values."

After preaching and then handing out a copy of the sermon to those in attendance and posting it to a UUC website, he said he was called before his battalion commander at Kandahar and told his sermon made Americans out "to be the bad guys" and "does not support the mission."

He was ordered to remove the sermon from the website, which he did, but following an investigation he said he was given an official reprimand in January 2013 for "politically inflammatory speech."

He then found himself being sent back to the U.S. with a "do not promote" evaluation and separated from active duty, he said in the Las Vegas speech. He challenged the Army's actions and, following a congressional inquiry by Sen. Kirsten Gillibrand, a Democrat from New York, he was reactivated and promoted to captain.

Antal said his congregation has been supportive of his action throughout. But he also said it is not just about him.

"It's about something bigger going on in our culture, and also about the relationship of the church and the government in this country," he said. "The church must reserve the right to speak for God against the nation and its policies under the Gospel. That's Christian language but I mean that in a very inclusive way, whether in a synagogue speaking for their God as they understand it under the Torah, or however you want to interpret it."

Also, he said, if the country claims to be one nation under God, then it has to accept that means not only under God's mercy and protection, but under God's judgement, as well.

"That flies in the face of American exceptionalism in the sense we can do whatever we want in the world," he said.

Editor's note: This story was updated to include the Army's explanation for a change in AR-165-1, Army regulations detailing the activities of the chaplain corps.

-- Bryant Jordan can be reached at Bryant.jordan@military.com. Follow him on Twitter at@BryantJordan.