Why did roosevelt choose portsmouth nh as the scene to settle the treaty of the russo-japanese war?

Because The Treaty of Portsmouth formally ended the Russo-Japanese War of 1904–05. The negotiations took place in August in Portsmouth, New Hampshire, and were brokered in part by U.S. President Theodore Roosevelt. The final agreement was signed in September of 1905, and it affirmed the Japanese presence in south Manchuria and Korea and ceded the southern half of the island of Sakhalin to Japan.