Britain may need to carry on air strikes against Islamic State (IS) for years to come - even if there is little prospect of delivering a knockout blow - a military think tank has warned.
A report by the Royal United Services Institute (RUSI) said that without a wider political settlement in Syria, the UK could have to sustain its military campaign against the extremists "over a period of several years".
Even then, the report said, it is possible - "perhaps even likely" - that the operation will end without having achieved a "decisive strategic effect".
David Cameron has signalled his renewed determination to seek Commons support for extending RAF air strikes against IS - currently restricted to Iraq - into Syria in the wake of the Paris terror attacks.
The report, by Professor Malcolm Chalmers, said it would be seen as an "important demonstration of solidarity with France" at a time when Britain's reputation as a reliable military partner was being undermined by the continued "parliamentary manoeuvring" over air strikes.
But while the case for joining the air strikes in Syria was now stronger than when the US-led coalition against IS was formed in 2014, it warned that the Government needed to be careful not to overstate their likely impact.
Although they had contributed to "second order objectives", such as protecting the Kurds in northern Syria, the report said that - given Britain's limited resources compared with the US - the addition of RAF war planes to the effort would not be "strategically transformative".
"In the absence of a wider political settlement in Syria, the UK's military campaign may need to be sustained over a period of several years," the report said.
"In these circumstances, it is possible - perhaps even likely - that the operation could end without achieving a decisive strategic effect."
The report said that it would be difficult to commit a higher level of air power to the conflict without eroding the RAF's ability to respond to other crises.
While the UK retained a "degree of influence" in the region, it said that politicians should not be under any illusion that Britain could be a central player in the diplomatic efforts to resolve the Syria crisis.
"The relatively limited nature of the UK role adds weight to the argument that it will be important to ensure that it does not over-commit resources to a protracted military campaign whose strategic objectives it has little ability to influence," it said.