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Nehemiah not only asked for permission to go to Jerusalem, he asked the king to give him letters of passage.  This was important in that time – it was almost like having diplomatic immunity.  Having this authority allowed Nehemiah to get past people who didn’t want him there.

6 The king said to me (the queen also was sitting beside him), “How long will you be gone, and when will you return?” So it pleased the king to send me, and I set him a date.
7 Then I said to the king, “If it pleases the king, let letters be given me to the governors of the province Beyond the River, that they may grant me passage until I arrive in Judah;
8 and a letter to Asaph, the keeper of the king’s forest, directing him to give me timber to make beams for the gates of the temple fortress, and for the wall of the city, and for the house that I shall occupy.” And the king granted me what I asked, for the gracious hand of my God was upon me.
9 Then I came to the governors of the province Beyond the River, and gave them the king’s letters. Now the king had sent officers of the army and cavalry with me.
10 When Sanballat the Horonite and Tobiah the Ammonite official heard this, it displeased them greatly that someone had come to seek the welfare of the people of Israel.  Neh 2:6-10 (NRSV)

Keep in mind that Sanballat and Tobiah represented people that had remained behind when the people of Judea were taken into captivity.  They didn’t welcome the prospect of a rebuilt Jerusalem occupied by Judah.  Nehemiah – in asking for the king’s authority – combined the authority of earthly powers with the ultimate authority of God’s power.