This one silly promise shows why George Osborne has messed up so badly

Britain's Chancellor George Osborne is watching how a policy he announced in his budget is tearing his party apart - and it's all his own fault.

In response to a planned £1.3 billion cut to welfare payments, former Tory leader Iain Duncan Smith (IDS) resigned from his position as Work and Pensions secretary and in effect forced Osborne to pull the proposed cuts.

This means that he has done an instant U-turn on his biggest single policy from this year's budget which was designed to reduce the deficit.

It's not like the Osborne didn't know that cutting money for the disabled would be unpopular, he trialed the policy ahead of the budget and just about every single newspaper article responded negatively. He's not doing it because he wants to do it, he's doing it because he has to do it.

Here's why.

It all starts with the run up to the last general election. It's hard to remember now, but it was seen as very unlikely that the Conservatives would win enough seats to form majority government. Everyone assumed that things would be much closer and that either the Conservatives or Labour would end up negotiating with the Lib Dems to form a coalition government.

With this in mind, Osborne said in the Conservative Party manifesto that he would cut £12 billion from the welfare bill. That's a massive amount to save, but Osborne thought he could use it to negotiate with the Lib Dems who would wants the cuts to be reduced.

Here's the promise that has caused him so much heartbreak.

The first sign that this promise was going to cause difficulty was in 2015's budget. Osborne tried to cut £4.4 billion in tax credits, a policy that turned out to be so unpopular with his own party he was forced to back down on it.

"We f----- up. We didn't mean to cut tax credits, it was a complete accident," an advisor to a cabinet minister told Business Insider UK last year.

This brings us on to the second thing Osborne put in the manifesto, that in hindsight probably wasn't too wise - the "triple lock" on pensions. Here it is:

Basically this meant, that Osborne can't cut the pension bill. To make the £12 billion in promised savings he has been left with two options - cut the housing benefit bill or cut the disability benefit bill.

The proposed cuts to disability benefits were the result of cold calculations - it's better to cut some money from disabled people than to risk ending up with an increase in homelessness.

The result of that one rash promise means Osborne has triggered a war within his own party and, following Monday's u-turn on the disability cuts, needs to find over £4 billion before the next election.