Sheikh Tamim bin Hamad Al-Thani, the youngest Arab head of state, succeeded his father Sheikh Hamad who abdicated on June 25.
It was a surprise move, especially coming at a time when the energy-rich Gulf state appeared to be at the peak of its political influence in the Arab world.
Just a week later, Egypt's army ousted president Mohamed Morsi and began a crackdown on his Muslim Brotherhood party and supporters.
"The collapse of the power of the Muslim Brotherhood in Egypt triggered the countdown for the end of Qatar's influence," said Antoine Basbous, head of the Paris-based Observatory of Arab Countries.
"This has negatively impacted the Islamists in Tunisia and militias close to the Muslim Brotherhood in Libya," he said.
The emirate has been extending political and financial support, as well as through its influential Al-Jazeera satellite television, to the Muslim Brotherhood in Egypt and Tunisia's Ennahda movement.
Qatar was even military involved in Libya's armed revolt which ousted Moamer Kadhafi.
In the Syrian conflict, Qatar has strongly backed the armed-rebellion against President Bashar al-Assad.
"The political influence that Qatar has with the opposition has weakened a lot, and Saudi Arabia now has the upper hand in the Syrian dossier," said a Syrian opposition figure, asking not to be named.
He pointed out that the head of the opposition National Coalition, Ahmed Jarba, elected in July, and interim opposition premier Ahmed Tomeh, selected earlier this month, are both close to Riyadh.
Tomeh replaced Ghassan Hitto, Qatar's candidate who resigned in July after failing to form an opposition government-in-exile.
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Qatar continues, however, to provide military aid to Islamist groups in Syria, in coordination with neighbouring Turkey, which is also a close ally of the Muslim Brotherhood, according to the Syrian opposition figure.
But a Qatari official insisted Doha's policies have not changed with the stepping down of the former emir along with his flamboyant premier and foreign minister, Sheikh Hamad bin Jassem Al-Thani.
"The foreign policies of Qatar have not changed. We do not support the Muslim Brotherhood and are at the same distance from all parties," said the official requesting anonymity.
As for Syria, "it was Qatar that paved the way for the international community to support the Syrian revolution," the official added.
But he noted that the new emir "has clear priorities on the internal level: this includes health, education and youths, and to pursue economic growth while controlling investments abroad."
Basbous said the new emir "does not want the impossible dream of the two Hamads"-- the former emir and his premier --- arguing that "taking leadership of the Arab world through the Muslim Brotherhood and Al-Jazeera channel is beyond the capabilities of Qatar."
However, Qatar remains a strong economic power in the region, thanks to its massive natural gas resources.
"Qatar's profile has been regionally reduced since the Egyptian coup, but it still has economic weight, regionally and internationally," said Gulf analyst Neil Partrick.
The new emir also seems "more cooperative than his father over trying to avoid support for the most dangerous armed jihadis in Syria," said Partrick. This "has helped improve relations with Saudi Arabia and may be reducing US suspicion of Qatar."
In a possible reflection of the new emir's priorities, Sheikh Tamim made his first visit abroad since becoming sovereign to Saudi Arabia, and the two countries have announced a border agreement.
But Qatar's relations with Riyadh, Abu Dhabi and Manama are "not good," said a Gulf official.
"The dispute over Egypt is big, and it is very big over political Islam," he told AFP, as Qatar continues to defend the Muslim Brotherhood while other Gulf monarchies support the new government in Egypt.
Some Gulf countries are also "upset over suspected Qatari support for Gulf opposition groups," the official said, pointing out this is a "red line" for the monarchies.