Here is an argument that crops up whenever the issue of the Ireland/UK land border is under discussion. If the UK, Ireland and the EU are unable to agree a way forward then the UK can simply announce that it has no intention of introducing controls or physical infrastructure on its side of the border. It will then be up to Ireland and the EU to decide what they want to do on their side. Thus if a hard border emerges, it won't be the UK's doing.
The argument illustrates a pattern of thinking which is not confined to the border issue, or even to Brexit. Its wrongness has a structure, so to speak, and that structure is at work in other contexts. It turns on treating outcomes (in this case, the 'invisibility' of the border) which arise from and require binding coordination of policy between states as if they could just as easily be achieved through unilateral decisions on the part of the same states.
Once that background assumption is accepted, then failure to achieve the outcome, even after one side has defected from policy coordination, can only be the result of unilateral choice on the part of one side or the other. If both sides want the same outcome, there is no reason to expect this to happen: contrariwise, if it does happen this can only be because at least one side wanted it to happen.
There are many desirable outcomes that can not emerge from unilateral, uncoordinated action by states even if all sides want the same thing. Examples include the existence of a common system of measurement for basic quantities such as mass, length, volume and time, being able to send mail from one country to another, being able to fly over and to land in other countries' territories and a myriad of other things which we take for granted. Most governments can see the advantage in having these things, but it is not enough for each to want the same outcome, because there is nothing a government can do acting on its own to secure it. They depend on governments mutually agreeing to limit their own freedom of action
and coordinate with each other.
The invisibility of the land border between the UK and Ireland is the product of particularly deep policy coordination. To state the obvious, invisibility is not the default state for international borders: it is a construction, and part of what goes into that construction is governments agreeing with each other to tie their own hands with regard to a range of policy areas, both international and -crucially- domestic. In order to avoid border controls, states need to address the reasons why border controls exist in the first place. Unilateral action can deal with some of those reasons, but not all.
Common membership of the single market and customs union has provided the coordination mechanism which ensures that most of the reasons why cross-border movement of goods needs to be controlled do not arise between the UK and Ireland, as between other EU states. Regulations, standards, tariffs and customs and VAT codes are the same on both sides of the border, within a shared governance and legal system. Absent this sort of deep, continuous and binding coordination, the requirement for border controls on the movement of goods inevitably re-emerges, without anyone having to do anything.
Governments can not create the conditions under which border controls are not needed by unilateral action- even if all governments would prefer there to be no controls. The reasons for controls won't go away just because nobody wants them, and to say that it would be 'up to' the EU and Ireland what to do in the event of no deal is simply to pretend those reasons don't exist. What governments can do unilaterally is create the conditions under which controls are needed, by withdrawing from the system of coordination that makes controls unnecessary, which is what the UK is doing.