(CNN) US President Donald Trump's policies and statements have ruffled the feathers of many world leaders since he took office.

On Friday, leaders from the world's biggest economies will meet at the G20 summit in Hamburg, Germany, where many of those public spats could be rekindled.

German Chancellor Angela Merkel has said she wants the summit to focus on key international issues such as climate change, free trade and an unfettered press -- issues that push up against Trump's positions and populist rhetoric.

Here's a look at some key issues that may be discussed at the summit -- and how Trump and G20 leaders have clashed over them.

CLIMATE CHANGE

All but three countries have signed the Paris Agreement, a landmark deal that asks every country to reduce their greenhouse emissions. In June, Trump pulled out of the agreement and joined Syria and Nicaragua in the small club of countries that have rejected the deal.

In a speech last week, Merkel spoke to Trump's worldview without directly naming him. She said, "We cannot wait until every last person on Earth has been convinced of the scientific proof."

Other G20 leaders haven't been shy to voice their discontent with his decision, including France's President Emmanuel Macron, Indian Prime Minister Narendra Modi, and Australian Prime Minister Malcolm Turnbull, who called Trump's withdrawal "disappointing but not at all surprising."

TRADE

Trump has forged ahead with his tough-on-trade campaign promises.

He has shown clear disdain for the North American Free Trade Agreement (NAFTA), saying the trade deal has unfairly robbed Americans of jobs.

In April, Trump announced he would stay in the deal but would negotiate its terms after speaking with his counterparts in Canada and Mexico.

Trump also fulfilled a promise to pull out of negotiations for the Trans-Pacific Partnership -- a 12-nation deal negotiated under Barack Obama. The TPP would have slashed tariffs for American imports and exports with those countries in exchange for attractive labor, environmental and intellectual-property protections for large businesses.

Following the decision, Chinese President Xi Jinping said his country would "remain committed to promoting free trade and investment through opening up and say no to protectionism."

Trump has criticized steel suppliers for cheating on steel prices. Experts fear a trade war could arise if Trump follows through with tariffs on those countries, which include, Canada, Mexico, Brazil the EU, Japan, and China.

NATO

Trump has repeatedly called the organization obsolete and has railed against members of the alliance for not spending the recommended 2% of gross domestic product on defense.

But he seemed to back away from disregarding the alliance in June by formally reiterating the US' commitment to the principle that an attack on one member is an attack on them all.

IMMIGRATION

Trump's move to block entry into the US of people from predominantly Muslim countries, which finally went forward last week , drew condemnation from many G20 leaders.

UK Prime Minister Theresa May called the ban "divisive and wrong."

Trump has also doubled down on his promise to build a Mexican border wall, since vowing that Mexico will pay for it. Mexico says it won't.

NORTH KOREA

Trump once told Bloomberg News, that if it would be appropriate, he'd be "honored" to meet North Korean leader Kim Jong Un. However, as tensions between Washington and Pyongyang escalated over Korea's nuclear ambitions and the detainment and death of an American student , Trump has gotten increasingly frustrated.

Trump has said that China isn't doing enough to pressure the regime.

Washington and Beijing have also clashed over the plan to deploy an anti-missile system in South Korea, something Russia also opposes.

SYRIA

The US and Russia say their forces are in Syria to fight ISIS, with Russia allied alongside President Bashar al-Assad's regime and the US working with groups that oppose both Assad and ISIS.

Tensions between Moscow and Washington have risen since the US launched an airstrike in Syria after Assad's forces staged a deadly chemical attack.

Last month the US shot down a Syrian warplane, prompting Russia to shut down a communication channel between the two countries.