The Munk leaders' debate on foreign policy, the first of its kind in Canadian history, was praised by many political observers as the most substantive of the campaign so far.

But the debate on pressing policy issues didn't prevent the leaders from going on the offensive in an effort to discredit their opponents ahead of the Oct. 19 vote. 

The three main party leaders squared off on the fight against ISIS, Syrian refugees, climate change and the Keystone XL pipeline and citizenship for convicted terrorists — and they each got in their fair share of jabs.

Stephen Harper's record was the focus of most of the attacks at Monday's event. But the frequent skirmishes between Liberal Leader Justin Trudeau and NDP Leader Tom Mulcair made it clear that the fight for left-of-centre voters has now taken centre stage.

Here are some key exchanges:

1. Harper vs. Trudeau on revoking citizenship of terrorists, Bill C-24

Liberal Leader Justin Trudeau: You devalue the citizenship of every Canadian in this place and in this country when you break down and make it conditional for anybody ... we have a rule of law in this country and you can't take away citizenship because you don't like what someone does.

Harper and Trudeau spar over C-24 at Munk debate0:45

Conservative Leader Stephen Harper: The individual in question, Mr. Trudeau, is convicted of planning the most heinous ...

Trudeau: ... And should be in jail.

Harper. A few blocks from here, (he) would have detonated bombs that would have been on a scale of 9/11. This country has every right to revoke the citizenship of an individual like that.

2. Bill C-51 — Anti-terrorism Act 

NDP Leader Tom Mulcair: Never forget that in Mr. Harper's failed Bill C-51, which was backed by Mr. Trudeau's Liberals, there was nothing on de-radicalization here at home.

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Liberal Leader Justin Trudeau, left, and Conservative Leader Stephen Harper at the Munk debate on foreign policy. Trudeau accused Harper of not doing enough to help in the Syrian refugee crisis. Harper says security of Canada has to be the top priority. (Andrew Vaughan/Canadian Press)

The NDP has a clear plan to bring in 2,500 more police officers across the country.

We know that we have to work with faith groups of all descriptions but you know, Mr. Harper always has one group in mind and he tends to finger-point and objectify one particular group.

He doesn't talk about houses of worship. He specifically refers to mosques and Muslims across Canada know how to interpret that for exactly what it is.

Trudeau: What Mr. Mulcair has done on C-51 is exactly what so many of us say that Mr. Harper has done, which is play the politics of fear. Now, with Mr. Harper, we know on C-51 he wants us to be afraid that there is a terrorist hiding behind every leaf and rock and he wants us to be afraid for he can say he is there to protect us. 

Mr. Mulcair is playing the politics of fear and division. Fear for environmental groups and First Nations, fear that we ripped up the Charter of Rights and Freedoms, and we know that's not true.

3. The Keystone pipeline

Harper: We far more often, than not, agree with our American friends. When we do not, we have to stand up for our interests and be very aggressive and vocal — pushing for Canada's best interests. 

Harper and Mulcair on Keystone XL0:46

Mulcair: Well, that's a wonderful idea. But when you actually need the approval of your colleagues for something you are pushing for, the last thing you should be doing is saying it's a complete no-brainer or you will take — or you won't take no for answer or you [say to] President Obama if you don't get it with this administration, you'll get it with the next administration. 

There's an old saying Mr. Harper, you catch more flies with honey than with vinegar. I think you were pouring vinegar by the gallon on the Americans. 

4. Trudeau on Trudeau

Mulcair: When you have convictions you have to have the courage to stand up for your convictions. The NDP took a stance against [Bill] C-51. The same way the NDP was the only one to stand up to Pierre Trudeau when he put Canadians in jail without charges. The NDP stood up against that. 

Trudeau: 'I am incredibly proud to be Pierre Elliott Trudeau's son'1:04

Trudeau: Throughout this campaign, in direct references and indirect references, both of these gentlemen have at various points attacked my father. Let me say very clearly, I am incredibly proud to be Pierre Elliot Trudeau's son. And I am incredibly lucky to have been raised with those values.

5. Climate change and reducing greenhouse gas emissions

HarperIn the electricity sector, we have proceeded in collaboration not with the United States but with our provinces on a position that is, frankly, going farther and faster than the United States. We will be the first country in the world to effectively shut down coal-fired, traditional coal-fired electricity, the biggest single source of emissions on the planet eliminated. In Canada, we will have the cleanest energy sector, and doing that without imposing taxes. 

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Trudeau: You will see the irony of standing here in Toronto and telling people in Ontario you somehow supported and aided the closing of coal-fired plants in Ontario. You and your government fought the Ontario government tooth and nail. The Ontario government was demonstrating it could reduce its emissions by the most significant degree of any jurisdiction by making a policy decision that you were no part of — you were blocking and ridiculing from Ottawa. 

6. The fight against ISIS

Mulcair: It is important to remember that this is not a NATO mission. This is not a United Nations mission.

When it was a question of going into Libya, under the United Nations duty to protect, the NDP voted for those airstrikes because it was a UN mission. When that started to morph into something completely different, we withdrew our support.

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So, the answer to your question is we understand that there will be times when we have to, either under the NATO Charter or under our international obligations at the UN, to use force and we won't shy away from that. But the real question here is that the only thing that we can do?

Harper: The other parties think we should not take direct military action against the Islamic State. This is a group that would slaughter literally millions of people in its wake, that has an intention, a stated intention, to attack — to launch terrorist attacks around the world, including against this country. And it has indicated it has the capacity to do that. We have a very clear reason for being there, supported by our allies in the international community. 

With files from Canadian Press