Times of Israel website founding editor David Horovitz told more than 800 guests at Monday's UJIA annual dinner that another Gaza war was inevitable and that people should be prepared to stand up for Israel.
Mr Horovitz warned that Hamas and other groups were in the midst of a "third phase" of efforts to destroy Israel, using terrorism alongside attempts to instigate international condemnation of Israeli actions.
"The climate here will not get easier, partly because Israel will get dragged into another conflict. And while we battle to survive, there will be more worldwide anti-Israel demonstrations. I imagine Britain's new leader of the opposition will be at the forefront of criticism. Again, there'll be a global media failure to show Hamas rocket fire and aggression."
To "prevent fair-minded people being misled by the haters", the Jewish community needed to be well informed and vocal. "Fight back through your area of expertise. You'll be protecting Israel and working towards a smart, knowledgeable climate for Britain. Our well-being is linked with Israel, so we might as well stand together."
At the central London dinner - whose proceeds are expected to be in the region of last year's £3.8 million - Mr Horovitz also condemned Western governments for not doing more to tackle the rise of Islamic extremism and for agreeing the recent nuclear deal with Iran.
There'll be a global media failure to show Hamas aggression
"I think this is a shameful moment in history and it will come back to haunt us all," he said. "I hope I'm wrong."
In its appeal video, UJIA focused on its work in providing educational scholarships to underprivileged Israelis and funding extended programmes in Israel for young British Jews.
In the video, Institute for Jewish Policy Research executive director Dr Jonathan Boyd, argued that for British Jewry, "everything we know about the organisational infrastructure, the leadership, the way this community functions, is showing signs of decline".
UJIA chief executive Michael Wegier said his charity "absolutely has the capacity to reverse this". He pointed out that those who returned from programmes in Israel "represent the very best of our capacity to build a strong, dynamic and creative future for British Jewry.
"If we are unable to generate the funds to get more young people on to better educational programmes, the impact on our community will be awful."
Professor Dan Ben-David of Tel Aviv University said that part of strengthening Israel's national security was "getting a first-world education to our children, so they can support a first-world nation, society and economy.
"If we act now, we're a young country, we're a dynamic country; the future is ours. If we don't, we lose the country. If we fail in the neighbourhood that we live in, we're dead.
"So we've got to get it right, and we've got to get it right now."
The dinner was attended by many of the community's spiritual and lay leaders.
More than 100 UJIA young patrons, youth movement leaders and Young UJIA committee members were also among the guests.