Order in number the events that started World War I.

Austria declared war on Serbia after Serbia rejects ultimatum.
Russia mobilized against Austria.
Italy signs secret treaty with Great Britain and joins the Allied Powers.
Great Britain reluctantly enters war after Germans violate Belgian neutrality.
Austria issued ultimatum to Serbia after Archduke's assassination.
Other European nations line up with Central Powers or Allied Powers.
Germany declared war on France and Russia.

Choices:
1
2
3
4
5
6
7

1. Austria issued ultimatum to Serbia after Archduke's assassination.

2. Serbia rejects ultimatum and Austria declares war.
3. Russia mobilizes against Austria.
4. Germany declares war on France and Russia.
5. Other European nations line up with Central Powers or Allied Powers.
6. Italy signs secret treaty with Great Britain and joins the Allied Powers.
7. Great Britain reluctantly enters war after Germans violate Belgian neutrality.

1. Archduke's assassination.

2. Austria issued ultimatum to Serbia.
3. Serbia rejects ultimatum, leading Austria to declare war.
4. Russia mobilizes against Austria.
5. Germany declares war on France and Russia.
6. Other European nations align with Central Powers or Allied Powers.
7. Great Britain reluctantly enters war after Germans violate Belgian neutrality.
8. Italy signs secret treaty with Great Britain and joins the Allied Powers.

Order: 1, 2, 3, 4, 5, 6, 7, 8.

To determine the order of events that started World War I, we need to consider the sequence in which these events occurred. Let's break down each event and match it with its corresponding number.

1. Austria issued an ultimatum to Serbia after the assassination of Archduke Franz Ferdinand.
2. Serbia rejects the ultimatum, leading to Austria declaring war on Serbia.
3. Russia mobilized its forces in response to Austria's declaration of war on Serbia.
4. Germany declared war on France and Russia after Russia's mobilization.
5. Other European nations aligned themselves with either the Central Powers or the Allied Powers.
6. Italy signed a secret treaty with Great Britain and joined the Allied Powers.
7. Great Britain reluctantly entered the war after Germany violated Belgian neutrality.

Now, let's assign each event its corresponding number based on the order in which they occurred:

1. Austria issued ultimatum to Serbia after Archduke's assassination. (Event 1)
2. Serbia rejects ultimatum, leading to Austria declaring war on Serbia. (Event 2)
3. Russia mobilized against Austria. (Event 3)
4. Germany declared war on France and Russia. (Event 4)
5. Other European nations line up with Central Powers or Allied Powers. (Event 5)
6. Italy signs secret treaty with Great Britain and joins the Allied Powers. (Event 6)
7. Great Britain reluctantly enters war after Germans violate Belgian neutrality. (Event 7)

Therefore, the order of events that started World War I is: 1, 2, 3, 4, 5, 6, 7.