By Humeyra Pamuk and Nick Tattersall

ISTANBUL (Reuters) - In the three and half months since a failed military coup, Turkey has sacked or suspended more than 110,000 people, launched a military incursion into Syria, and repeatedly threatened to do the same in Iraq.

To cheers from his supporters, President Tayyip Erdogan, evoking the glories of Turkey's Ottoman past, has vowed to root out enemies at home and abroad, from followers of the cleric he blames for the coup attempt, to Kurdish militants and Islamic State jihadists.

The unprecedented crackdown at home and his bellicose stance on the world stage have alarmed Western and some regional allies, who fear the NATO member and EU candidate nation is becoming an ever more unpredictable partner, and one over which they have decreasing leverage.

In the latest purge, police on Monday detained the editor and senior staff of the Cumhuriyet newspaper - one of few outlets still critical of Erdogan - over its alleged support for the July putsch. A senior EU politician described it as crossing a red line against freedom of expression, while the U.S. State Department expressed deep concern.

Erdogan is riding a wave of patriotism as the ruling AK Party he founded seeks constitutional change to move Turkey to a fully presidential system which would give him greater executive powers.

"What's happening domestically and in terms of Turkey's foreign policy are a political tactic to keep solid the alliance between the base of the AKP and the nationalists," said Sinan Ulgen, a former Turkish diplomat and analyst at Carnegie Europe.

"This alliance is keen on harsh policies on the Kurdish issue, looks to be in favor of reinstating the death penalty, and we can't really say they regard the preservation of freedom of speech and of the media very highly," he told Reuters.

There is no sign of any easing in Turkish policy at home or abroad, given the need to ensure nationalist support for the constitutional changes that Erdogan and the ruling party want to result from a referendum, which AKP officials have said could be held next spring.

The nationalist MHP opposition party, many of its fervently patriotic members supportive of Erdogan's stance since the coup, has indicated it could back the AKP in parliament as it seeks support for the referendum on the presidential system.

FRAUGHT WITH DANGER

But while the purges at home may continue - Monday's action at Cumhuriyet came a day after 10,000 more civil servants were dismissed and 15 media outlets shut down - the strident words on foreign policy may not translate so readily into action.

Erdogan warned this month that Turkey "will not wait until the blade is against our bone" in going after its enemies abroad and has hinted at a possible incursion into Iraq if a U.S.-backed assault against Islamic State in the city of Mosul causes sectarian strife which threatens Turkey's borders.

Frustrated that it has not been more involved in the Mosul operation, Sunni Muslim Turkey says it has a responsibility to protect ethnic Turkmens and Sunni Arabs in the area, once part of the Ottoman empire. It fears Shi'ite militias, which on Saturday joined the offensive west of Mosul, will provoke ethnic bloodletting.

A Turkish ground operation would, however, be fraught with danger, risking embroiling its military in a third front as it pursues an offensive against Islamic State in Syria and against Kurdish PKK militants in its own southeast.

"Turkey is an impactful actor within its region, but has to correctly appreciate its weight," said Aydin Selcen, a retired diplomat who was Turkey's first consul general in Erbil, the capital of northern Iraq's autonomous Kurdish region.

"History is like a huge supermarket where you can find what you want. You can choose a historical perspective created to rally the masses. But you can neither build a foreign policy nor a military strategy based on that," he told Reuters, alluding to the Ottoman references in Erdogan's speeches.

"YET ANOTHER RED LINE"

Carnegie Europe's Ulgen also doubted Turkey would embark on measures such as a ground offensive in Iraq, casting it as rhetoric for a domestic audience. But he noted such words risked undermining Turkey's credibility and had unintended consequences for foreign policy.

"Turkey now finds itself facing tougher opposition abroad and is losing its ability to strike alliances, not only with its traditional partners but with regional actors as well," he said.

The crackdown at home is doing little to help.

European Parliament President Martin Schulz wrote on Twitter that the detentions at Cumhuriyet marked the crossing of 'yet another red-line' against freedom of expression. "The ongoing massive purge seems motivated by political considerations, rather than legal and security rationale," he said.

The Istanbul prosecutor's office said the staff at the paper were suspected of committing crimes on behalf of Kurdish militants and the network of Fethullah Gulen, the U.S.-based cleric Ankara accuses of masterminding the July 15 coup attempt.

Government spokesman Numan Kurtulmus declined to comment on the legal action at Cumhuriyet.

But more broadly, officials say such measures are justified by the threat posed by the putsch, in which more than 240 people were killed when rogue soldiers commandeered tanks and fighter jets, opening fire on parliament and other key buildings.

Since the attempted coup, 170 newspapers, magazines, television stations and news agencies have been shut down, leaving 2,500 journalists unemployed, the Turkish journalists' association said in a statement protesting the detentions.

Opposition groups say the purges are being used to silence all dissent.

"We are facing a new phase in the coordinated oppression managed by the AKP headquarters to ensure no opposition remains," Selahattin Demirtas, head of the pro-Kurdish Peoples' Democratic Party), told reporters.

(Additional reporting by Tuvan Gumrukcu in Ankara; Writing by Nick Tattersall)