Childhood friends Mohammed Nahin Ahmed and Yusuf Zubair Sarwar, both 22, spent eight months in Syria last year after contacting Islamic extremists from the UK.
The men from Birmingham were arrested at Heathrow by West Midlands police's counter-terrorism unit on their return in January after their families had put pressure on them to return to Britain.
Ahmed, a former postman who was born in Bangladesh, moved to Britain as a child, while Sarwar, who is of Pakistani descent, was born in Britain.
Police believe Ahmed and Sarwar fought with the al-Nusra Front, which is a jihadist group affiliated with al-Qaeda.
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Their trial had been due to start but, at Woolwich Crown Court in London, they each admitted one count of engaging in conduct in preparation of terrorism acts, contrary to section five of the Terrorism Act.
A date for sentencing is expected to be confirmed in a few weeks, the BBC reported.
Police said they were alerted to the case after Sarwar's parents contacted them in May 2013 to say he was missing.
He also left instructions to cancel his mobile phone contract and money to settle outstanding debts.
Police said Ahmed had "considered going to Yemen and had sought advice from a fighter in Syria and from extremists in Denmark and Sweden".
The men bought one-way tickets to Turkey then crossed the border to Syria - and when they got back officers were "waiting to arrest them", police said.
"Traces of military grade explosives were found on their clothing and pictures on their camera showed them brandishing weapons," they said.
Assistant Chief Constable Marcus Beale said the men had gone to "considerable lengths" to hide their plans from their families, and urged people to tell the police if they suspected a family member was planning to travel to Syria.