Corbyn claims Britain's foreign policy is leading to blowback from the Islamic world, resulting in terrorist attacks on Britain.
I would say that's a convenient (to the right wingers) over-simplification, in that it portrays him as putting forward an inane tit-for-tat type of explanation for terrorism where it's our "karma" in some way, or a moral thing - we do bad and they do bad back, whereas what he has actually said is that ill-judged and ill-thought-out interventions in Islamic countries - Afghanistan, Iraq, Libya - have led to vast ungovernable spaces within them where the jihadis have organised and recruited. (That is is, a practical explanation, and not one he dreamed up himself). There does seem to be a wilful tendency across the media to portray Corbyn’s supporters, Corbyn himself and his policies as if they were mindless or worse.
Many conservative types find that view unacceptable and unpatriotic.
Many "conservative types" have opposed questioning of the political judgements that led to military disasters such as Gallipoli, Suez, etc, and they used the "unpatriotic" smear, as if only they held the right to decide what that means.
At the end of the day, I don't think most Brits support unprovoked military intervention into the Islamic world.
I think most Brits are reluctant to attack anyone just for the sake of it, but would support necessary action that actually protects and safeguards us.
I personally have questions, though, about how he views a couple foreign powers, Israel and Russia.
Israel (July 2015 statement)
Support a viable peace process between Israelis and Palestinians, based on internationally recognised (1967) borders.
Palestinian Statehood: Reaffirm the Labour party’s commitment to the recognition of a safe and viable Palestinian State alongside a safe and viable Israel.
Human Rights: Oppose violations of international human rights law, in particular the detention of children and detention of political prisoners without trial.
The Wall: Oppose the continued construction of the Separation Wall on Palestinian land, a direct contravention of international law.
The Blockade: End the siege on Gaza and ensure the free flow of aid and trade
Illegal Settlements: Call for a complete freeze on illegal settlement growth in order to save any hope for a viable two state solution, and end all trade and investment with illegal Israeli settlements on occupied Palestinian territory.
He is not an admirer or supporter of Putin’s foreign policy, or of Russian or of anybody else’s expansion. He has called Russian intervention in Syria "inexcusable". He is opposed to "... both NATO expansion and Russian expansion—one leads to another, and one reflects the other.’
There is an interesting opinion piece by Jonathan Rosenhead in the Israeli Ha'aretz newspaper, published just before Britain's General Election:
I'm a British Jew, and I Don't Fear a Corbyn Victory. I'd Welcome It
The caricature of Jeremy Corbyn as a tool of Trotskyites, a lover of dictators and a shill of anti-Semites is unhinged and wrong