'Start Here': 1,000 more troops to the Middle East, and a 'war zone' in Texas Here's what you need to know to start your day.

It's Tuesday, June 18, 2019. Let's start here.

1. 'Nuclear extortion'

The U.S. is sending an additional 1,000 troops to the Middle East amid heightened tensions with Iran, which the U.S. has accused of carrying out last week's attack on two oil tankers in the Gulf of Oman.

Iran also is threatening to exceed its uranium stockpile limit, a violation of the 2015 nuclear deal, unless European leaders provide relief from crippling sanctions.

U.S. State Department spokesperson Morgan Ortagus responded on Monday by demanding that Iran follow the terms of the agreement, even though the U.S. withdrew from it last year.

"We should not," she said, "yield to nuclear extortion by the Iranian regime."

"They say they're for defensive measures," ABC News Chief Global Affairs Correspondent Martha Raddatz says of U.S. officials allocating more troops to the region. "They want more intelligence, they want more surveillance ... because of this threat that they say is credible."

2. Courthouse shooting

A masked gunman wearing tactical gear opened fire outside a federal courthouse in Dallas on Monday, sending bystanders running for cover as he exchanged gunfire with federal authorities.

"The shots, they just kept going," ABC News' Marcus Moore reports from Dallas. "One witness told me it just seemed like it would not stop. It was really like being in a war zone."

Authorities are trying to determine a motive after the suspect, 22-year-old Brian Isaack Clyde, was killed in the gun battle, according to the FBI.

3. 'Double jeopardy'

The Supreme Court has upheld a constitutional "double jeopardy" rule that allows someone to be charged by state and federal governments for the same crime.

Court observers had been watching the case, as the justices' decision on Monday could have implications for presidential pardon power, particularly if President Donald Trump decides to pardon his former campaign chairman, Paul Manafort, according to ABC News' Devin Dwyer.

"If the court had ended this exception, it could've, in effect, expanded that power, allowed the president of the United States to pardon somebody and be immune to prosecution," Dwyer tells us.

In another decision on Monday, the Supreme Court wiped away a lower court ruling against two Christian bakers in Oregon who refused to make a wedding cake for a gay couple because of their religious beliefs. The justices sent the dispute back to the Sixth Circuit to review.

4. Zuckbucks

Facebook is reportedly set to enter the world of cryptocurrency with its own digital coin called Libra.

The project is expected to make a large profit for the social media giant, predicts Forbes contributor and appointee to the Wyoming legislature's Blockchain Task Force Caitlin Long tells us. But there could also be more backlash for Facebook.

"Grab the popcorn," she says on "Start Here." "There are going to be all kinds of crosswinds with regard to fairness, with regard to how Facebook is protecting data, with regard to government wanting to get access to Facebook's data for tax collection purposes and financial surveillance purposes."

"Start Here," ABC News' flagship podcast, offers a straightforward look at the day's top stories in 20 minutes. Listen for free every weekday on Apple Podcasts, Google Podcasts, iHeartRadio, Spotify, Stitcher, TuneIn or the ABC News app. Follow @StartHereABC on Twitter, Facebook and Instagram for exclusive content and show updates.

Elsewhere:

'As fast as they come in': The president tweets about immigration.

'Honestly, it hasn't done anything to help us': A Phoenix couple roughed up and threatened by local cops discuss apologies they've received from the mayor and police chief.

From our friends at FiveThirtyEight:

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Doff your cap:

This 10-year-old just made rockclimbing history.

On June 12, Selah Schneiter became the youngest climber to make it to the top of "The Nose" route on El Capitan, one of the most challenging and infamous vertical formations in Yosemite National Park.

"Our big motto was, 'How do you eat an elephant?' Small bites," she told ABC affiliate KFSN. "One pitch at a time, one move at a time, one day at a time."

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She was joined by her dad, Mike Schneiter, and longtime family friend Mark Regier. Their attempt to climb the full route took place over five days, including camping overnight on the slab.

"She's all about wanting other kids to be more active, whether it's biking, climbing, skiing," her father added. "She always wants more kids to be active and not be on their phones, and I think that's ultimately some of our motivation to talk about it, because it's really near and dear to her."