A defence attaché is a member of the military serving in an overseas embassy, representing their country's defence abroad.
The term 'defence attaché' covers personnel from all branches of the military, and those in the role have diplomatic immunity and status.
It is the defence attaché's job to protect, develop and promote the defence interests of their county in the nation they are assigned, as well as in bilateral military and defence relations.
Simon Diggins said UK defence attaché in Kabul, Afghanistan, between 2008 and 2010, told Forces News the role sits "between the Ministry of Defence (MOD) and the Foreign Office".
"If something happens - dramatic, good or bad, and the Ukrainian government need a very quick response from from the British, they've got people on the ground who they can call pretty quickly who will be able to get to their headquarters, get to a meeting and receive that information quickly," he said.
"The defence attaché, they're providing defence advice to the ambassador - who is representing the whole of the Government as well as being a forward element of the Ministry of Defence itself," he said.
The UK's attaché to Ukraine is preparing for a return to Kyiv after leaving the country following Russia's invasion.
He added the ability of the UK and Ukraine to "respond quickly to events" will be enhanced by the redeployment of the defence attaché.
Mr Diggins also said having people on the ground in a country helps both the UK and the nation that a defence attaché is operating in.
"They've got people on the ground who they can call pretty quickly who will be able to get to their headquarters, get to a meeting and receive that information quickly," he said.
"It doesn't have to be filtered, go through an email channel, or anything like that.
"So again, that ability to respond quickly to events has got to be enhanced by having somebody on the ground."
He also said that having a defence attaché in a country gets you a "personal touch" that is lost when communicating from far away.
During two years in Kabul Mr Diggins said he was able to bring both "continuity" and "good local knowledge" to the role.
"What I was able to bring to that was continuity firstly and, secondly, good local knowledge, not just of the situation - though I can never really claim to have a complete handle on it because it was so complex and so fast moving - but understanding the personalities and also taking a slightly strategic picture and say what it is going to look like in two, three, four, five years' time and offer that as a perspective back to [the UK]."
Some defence attachés are deployed to work on specific issues, like migration, and they can also serve as part of a military mission with organisations such as NATO, the EU, or the UN.
If serving on such military deployments, defence attachés usually operate as the head of the mission or a military adviser - with these assignments usually multilateral.