If and when Israel embarks on a major offensive against Hamas in the Gaza Strip, a lot of people — watching from a safe distance — are going to be very cross. Human Rights Watch will be up in arms; Amnesty will be outraged. The BBC, CNN and Al Jazeera will show harrowing pictures (real and fabricated) of Israel’s “disproportionate” action. My Facebook feed will be full of friends expressing horror at Israeli air strikes in Gaza, and #GazaUnderAttack will be trending on Twitter. All this, to the tune of another, repetitive chorus of Restraint from the diplomatic choirs of foreign governments.

So to everyone who, as is looking increasingly inevitable, is soon going to be tutting and sighing and condemning Israel for finally retaliating against Hamas, I have one message:

Speak now, or forever hold your peace.

I say this as someone who wants peace to reign over this land, which is what I imagine you want too (right?). I say this as a friend, and as someone whose stomach churns when he sees pictures of injured children and whose heart breaks when he sees pictures of them dead. I don’t want another Cast Lead or Pillar of Defence, so take this as a piece of friendly advice, from one person who cares desperately about human rights to — presumably — another.

Here’s a summary of what has happened in Israel tonight. Between 8pm and 9pm, Hamas fired one rocket a minute at Israel, aiming to murder as many civilians as possible. Residents of the south have had fifteen seconds to flee for cover. Sirens have been heard as far as Jerusalem. Hamas is threatening to strike Tel Aviv. Hospitals are moving babies into bomb shelters. This comes in the wake of an escalation in recent days, which you probably didn’t hear about, in which a nursery sustained a direct hit. Israel has given Hamas a 48-hour ultimatum, but it expired 48 hours ago. Don’t say you weren’t warned.

So here’s why I ask you, kindly, to speak now or forever hold your silence. Because if you fail to condemn Palestinian terrorism, and are saving your outrage for Israel’s response, you will have no moral right to complain. When you say you care about innocent civilians, no one in Israel will believe you. When you say you want peace, Israelis will laugh in your face. When you say that violence is not the solution, many in Israel will agree with you, but everyone will want to know where you were yesterday, and why you only just remembered you care about crying children when it’s Palestinian children shedding the tears.

And here’s why this matters: because if you don’t speak now, if you don’t complain and protest with all your might now, you will have zero credibility with the very people whose behaviour you are trying to influence. You won’t be able to convince Israel to hold its fire when it’s clear that you don’t care about its people. Why should they listen to you? Who do you think you are? You will never be able to convince people that your solution is the best if they perceive you as a pathetic rent-a-gob at best or an apologist for global jihad at worst.

Your words of indignation will assuage your conscience and give you the glow of moral smugness, but they will be completely ineffectual.

So take your ‘Stop the War’ signs and march straight to Trafalgar Square, or I can tell you where you can shove them.

Speak now, and speak loudly, or forever hold your silence.

Because right now, over the wailing of the sirens and shouts of “everyone to the bomb shelters!”, it is the echo of your own moral vacuousness and insincerity that reverberates most loudly.

Read, share, repeat, and — of course — retweet.