There seems to be no doubt in May's mind that leaving the EU will make the UK less secure:

"So my judgement, as Home Secretary, is that remaining a member of the European Union means we will be more secure from crime and terrorism". Theresa May 25th April 2016.

"In security terms a failure to reach agreement would mean our cooperation in the fight against crime and terrorism would be weakened". Theresa May 29th March 2017  invoking Article 50.

This is not a good stance on which to build a case for leaving the EU and cobbling together building a strong and stable government over the next 5 years - but it gets worse. On the causes of terrorism there seems to be no coherent thinking.

Boris Johnson, now foreign secretary, seems to think that British wars overseas have increased terrorism: "Isn't it possible that things like the Iraq war did not create the problem of murderous Islamic fundamentalists, though the war has unquestionably sharpened the resentments felt by such people in this country and given them a new pretext?"

In an 11-minute interview with by Krishnan Guru-Murthy on Channel 4 News, defence secretary Michael Fallon condemned that comment. This statement came after Jeremy Corbyn's speech on terrorism. 

Still with this?

Prime Minister: Leaving the EU will make the UK less secure.

Foreign Secretary: Overseas wars encourage terrorism

Defence Secretary: Oh no they don't

cue Pantomine Horse?

By the way...

you can read the full text of Jeremy Corbyn's speech on 26th May but the parts that seem to have stirred things up are:

"We will also change what we do abroad. Many experts, including professionals in our intelligence and security services have pointed to the connections between wars our government has supported or fought in other countries, such as Libya, and terrorism here at home."

and

"I want to assure you [our Armed Services, Foreign Office and International Development professionals] that, under my leadership, you will only be deployed abroad when there is a clear need and only when there is a plan and you have the resources to do your job to secure an outcome that delivers lasting peace."