New Delhi: When Sri Lanka unexpectedly turfed out President Mahinda Rajapaksa in an election this month, it was the biggest setback in decades for China's expansion into South Asia - and a remarkable diplomatic victory for India.

Despite New Delhi's protestations, diplomats and politicians in the region say India played a role in organising the opposition against pro-China Rajapaksa.

In September, President Rajapaksa allowed a Chinese submarine to dock in Colombo, without informing India, as it was bound to under an existing agreement.

His successor, President Maithripala Sirisena, has said India is the "first, main concern" of his foreign policy and that he will review all projects awarded to Chinese firms, including a sea reclamation development in Colombo that would give Beijing a strategic toehold on India's doorstep.

India has pushed back against China elsewhere in the region since Prime Minister Narendra Modi took office in May, improving ties with Japan and Vietnam, both locked in territorial disputes with Beijing, and contesting a port project in Bangladesh that could otherwise have been a cakewalk for China.

The new robust diplomacy, which Modi calls "Act East", has delighted Washington, which has been nudging India for years to dovetail with the U.S. strategic pivot toward the region.

When President Barack Obama makes a landmark visit to India starting Sunday, he will be the chief guest at New Delhi's showpiece Republic Day military parade, and rarely for a presidential trip, is not scheduled to visit any other country before returning to Washington.

"What is appealing to me and my colleagues is the fact that Prime Minister Modi has undertaken to build from what has been a 'Look East' policy to an 'Act East' policy," U.S. Assistant Secretary of State for East Asia and the Pacific Daniel Russel said in Washington last month.

However, the bonhomie has limits. Tricky conflicts over trade and intellectual property hold back business.

But Mr Modi's policies mark a departure from India's traditional non-aligned approach to foreign power blocs.

"Having the U.S. president at the Republic Day celebration is a good thing, he is blessing Modi," said Mohan Guruswamy, of the Centre for Policy Alternatives, a think-tank.

"And that is a lesson to the Chinese that you have to mend your fences with us."