Campaign volunteers

.

Decision-making should involve at least an attempt of weighing up pro and contra; of trying to get the cost/benefit ratio right. But not all costs matter. There are always both relevant and irrelevant costs; sunk costs are irrelevant costs, for example.

Now let's assume some soldiers or reservists volunteer for a campaign. This would be an individual-level decision, and a government's decision could then be limited to the four options

(1) government-run expedition

(2) covert government-run expedition

(3) ignoring it

(4) criminal prosecution after the fact*

That's a very different decision than a political 'war or peace' decision involving a military only.

The price of casualties amongst the troops would probably need to be appraised differently than in the plain 'war or peace' case, as the volunteers would have made that decision for themselves already, based on their individual preferences which are unknown to the government.**

The flow of legitimacy is upwards from the voter to the candidates in a democracy, because the voters know their preferences and add their vote to the overall pool of votes.
One might reason that the flow of legitimacy about the decision whether individuals can go to war cannot be in the reverse direction because again the voters (volunteers) act based on their preferences which are unknown to everyone else. So nobody else could make a better decision for them.
An exception would be if their action would harm the rest of the society, such as through worsened foreign relations, retributions or the fiscal costs of a (para)military operation.

The government can thus not claim to make a better decision on behalf of the volunteers themselves - it can only claim to make a better decision on behalf of other citizens who might be affected.

Volunteer forces were somewhat popular in the past, with the (case 2) American Volunteer Group and (case 3) Eagle Squadrons being examples. There are also thousands of case (3) and (4) examples of so-called jihadists right now and the insurrection in the Eastern Ukraine may be a case (2) on more than only the Ukrainian side.

Mercenaries and volunteer foreign fighters aren't merely an approach to feign neutrality of their country's government; they do require a very different ethical and practical decision-making on part of the government.

Plausible deniability is important though, as the United Nations' General Assembly defined aggression in 1974, including this quote:

(g) The sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to the acts listed above, or its substantial involvement therein.

That is, unless you are sure some UNSC veto will protect your government from official international repercussions. Plausible deniability doesn't help against unofficial backlash anyway.

The employment of actual volunteer contingents could become an important foreign policy tool in the decades to come when governments attempt to disassociate themselves from backlash-inducing meddling. It may also already be a major challenge, as governments strive to suppress unwanted mercenary and foreign fighter activities.

S O

*: Some countries outlaw mercenary activity in itself.

**: This is a rather important facet of economic theory. Some perfect outcomes are impossible in practice because of the inability to use a god-like knowledge about the people's preferences. You need to know preferences for a perfect allocation of resources, for example. Markets and democratic elections or plebiscites are tools to reach outcomes based on at least a decentralised consideration of preferences.

.