Prime Minister Justin Trudeau was accused of "stepping back" from the coalition fight against ISIS as a feisty first question period kicked off today.

Interim Conservative Leader Rona Ambrose was quick to drop the gloves as she asked why Canada is ending airstrikes against the militant group in Iraq and Syria when U.S. President Barack Obama called the aerial bombing campaign a "key pillar" of the mission.

"Why is the prime minister stepping back from the fight when our allies are stepping up?" she asked.

Ambrose pressed Trudeau to concede he is more committed to his own "ideology" than he is to Canada's allies.

Describing ISIS as a "death cult" that sells women and men into slavery and targets religious minorities, gays and lesbians, Ambrose asked just how bad it would have to be for Trudeau to leave Canada's warplanes in the theatre.

Interim leader of the Opposition Rona Ambrose comments on the government's speech from the throne on Parliament Hill in Ottawa on Monday. (Adrian Wyld/Canadian Press)

Trudeau said Canada will continue be a strong partner in the coalition. He has promised to expand the number of Canadian troops who will train local forces, and to continue with humanitarian assistance including the resettlement of Syrian refugees.

"The question has always been, how best to engage," he said.

Trudeau and his cabinet ministers faced a range of questions, from the feasibility of tax cuts for the middle class, to refugee resettlement.

Mulcair asks about climate change

NDP Leader Tom Mulcair used his time on the floor to grill Trudeau on the government's plan and targets to reduce greenhouse gas emissions.

"Canadians are proud to see a change in tone on climate change, but they're also hoping the change in tone will bring about a concrete change of direction," he said. "Once [the government] finally decides on Canada's targets, will it make them binding by enshrining them into a climate change accountability law here at home?"

Trudeau shot back that Mulcair "forgets from time to time" that Canada is a federation with 10 provinces that have different approaches and different requirements.

"We have committed to sit down and engage with those provinces, listen to them and work out not just targets but a plan that is going to ensure Canada meets its international and domestic obligations to reduce climate emissions," he said.

While all parties had committed to adopting a more conciliatory, constructive tone, in Parliament the first question period featured the traditional heckling and partisan barbs.

But there was a touching moment of humanity as well, when longtime Liberal MP Mauril Bélanger received a standing ovation from MPs from all parties before and after asking a question, as his colleague Larry Bagnell fought back tears. Bélanger recently disclosed he has been diagnosed with amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.

New dynamic in House

The new House of Commons has a dramatically new dynamic, with the Liberals in government, the Conservatives in opposition and the NDP relegated to third spot. There are also 30 more MPs in the House, bringing the total count to 338.

Melanee Thomas, assistant professor of political science at the University of Calgary, said this session of Parliament will see much attention on Trudeau's performance in question period, which has been not "great" in the past. She said it will be interesting to watch how Ambrose stacks up against Mulcair as the Opposition leader.

"The former is rather untested in a major question period role; the latter was the prosecutor of Parliament during the last session," she said.

There are a number of key political matchups in the new Parliament, with rookie ministers facing questions from seasoned former Conservative cabinet members.

Conservative finance critic Lisa Raitt prefaced her question to Finance Minister Bill Morneau by saying they would have "a lot of fun" in the weeks and months ahead, before challenging him over "broken promises" and "flawed" plans.

Morneau explained that the Liberals "inherited" a fiscal situation that was more challenging than foreseen in the budget.