WASHINGTON (Reuters) - Disputes between U.S. Republicans and Democrats over President Donald Trump’s proposed border wall, Space Force and his ability to launch a war with Iran are among the biggest obstacles Congress faces in passing a massive annual defense policy bill.

FILE PHOTO: A soldier assigned to the U.S. Army's 3rd Brigade, 101st Airborne Division (Air Assault), participates in a training during a tactical leader's exercise at Eglin Air Force Base, Florida, U.S., August 27, 2019. REUTERS/Bryan Woolston

Members of the House of Representatives and Senate formally launched on Thursday this year’s negotiations on the National Defense Authorization Act, or NDAA, the $700 billion bill that sets policy for the Pentagon.

The NDAA has been signed into law every year since 1961, one of the few major pieces of legislation that always gets through Congress despite deep divides between the two parties.

Because it is a rare “must-pass” bill, the NDAA becomes a vehicle for a broad range of policy measures, as well as determining everything from military pay levels to which ships or aircraft will be modernized, purchased or discontinued.

Passing the fiscal 2020 version of the legislation could be difficult, given angry disputes between Republican President Donald Trump and congressional Democrats as well as some of his fellow Republicans.

This is the first year of Trump’s presidency that Democrats have controlled one house of Congress. Trump has already threatened to veto the version of the measure passed by the Democratic-controlled House, partly over his desire to build a wall on the border with Mexico.

The House and the Republican-controlled Senate have each passed their own versions of the NDAA. Thursday marked the start of the conference, in which lawmakers negotiate a compromise.

Representative Adam Smith, chairman of the House Armed Services Committee, said conference members had not yet made decisions on the major sticking points, including whether they would even try to address border wall funding or leave it for the Defense Appropriations bill.

“I appreciate the questions, but we’re not going to tell you how we’re going to have an outcome here at this press conference,” Smith said.

Lawmakers are also divided over an amendment that would bar Trump from taking military action against Iran without obtaining Congress’ approval. That issue became more important this week after Washington and some allies blamed the Islamic Republic for Saturday’s attack on Saudi oil facilities.

Democrats are also balking at Trump’s desire to create a Space Force, a new branch of the military for space, which could cost billions of dollars.

Trump’s promise to build a border wall was a feature of his 2016 presidential campaign, and has been a theme of his bid for re-election in 2020. The House and Senate voted against Trump’s declaration of a national emergency to shift money to the wall, calling it a violation of the constitutional mandate that Congress, not the president, controls spending.

However, there was too little support from congressional Republicans to overcome Trump’s veto.