<p>A very short answer is <strong>because Eren doesn't know how to.</strong> </p>

<p>Spoilers below:</p>

<blockquote class="spoiler">
  <p> Eren has what that has been called as <strong>"The Co-ordinate"</strong> which was passed to him from Freida Reiss through his father. This is the ability of the Founding Titan which allowed them control over the titans and memory manipulation.<br> We do not yet know how exactly this power works. The real limit of this power is unknown, but as it can be observed, it extends to the point of making Titans perform actions that threaten their own lives. It was also mentioned by Rod Reiss that the power is so strong, it has the potential to wipe out all the Titans if the user is able to wield it to its full strength. </p>
</blockquote>

<p>Then why it was Eren was unable to use it while he had contact with a royal bloodline, the reason is that Eren has only shown this power <em>under very rare and unique circumstances</em>. One can argue that the danger to his life was one such circumstance, but unless the writer clarfies the exact nature of this power we don't know the limitations.</p>

<p>For more information, with <strong>HEAVY SPOILERS</strong> refer to : <a href="http://attackontitan.wikia.com/wiki/Founding_Titan" rel="nofollow noreferrer">Wikia : Founding Titan</a></p>

CC BY-SA 3.0
2017-05-24T11:19:47.353Z
Thank you! The detail and coherence of your answer is just magnificient.
Ryota
CC BY-SA 3.0
2017-05-24T09:35:10.463Z
2017-05-24T09:35:10.463Z
Arcane
<p>Heavy spoilers under spoiler section (until manga chapter 89 including)</p>

<blockquote class="spoiler">
  <p> Eren is able to use Founding Titan power only when in direct physical contact with person of royal blood. He comes to this conclusion in chapter 89, which also explains events of chapter 50 (titans got Eren's order, while he had direct contact with Dina Fritz's titan), and chapter 62 (when Eren was able to see memories, stored in his titan, while Historia was touching him).</p>
</blockquote>

<p>Now, regarding your question:</p>

<blockquote class="spoiler">
  <p> Actual combat with Rod Reiss in his titan form has started somewhere around chapter 68, which was a lot earlier. So, basically, on that time, he had no idea, how to activate this power.</p>
</blockquote>

CC BY-SA 3.0
2017-05-25T11:05:41.987Z
The OP i believe understands the restriction of the Royal Family, but is asking about a specific use of the Founding Titan's power even when he had Historia near him.
Arcane
CC BY-SA 3.0
2017-05-25T11:57:51.630Z
@Arcane well, I've missed the part, where OP mentions, that he knows that royal blood is required. Yet, I've referenced chapter, where Eren actually came to this conclusion. Which is far later than chapters where Eren and Co were fighting Rod. But, I suppose, I will update answer with this info.
lentinant
CC BY-SA 3.0
2017-05-25T19:22:05.443Z
First of all, I thought that it is enough for a Royal Family member to be around Eren in order for him to use the "Titan Control" power. But we can deduce that he actually has to be touched by them. That was absolutely not the case during the battle against Rod Reiss' titan. However, shortly after unintentionally commanding titans to kill Dina, he commanded them to eat Reiner. Thus, we can deduce that his power lasts for a specific amount of time after touching them. But these are only theories. Just as Arcane said, Eren's powers are still a mistery to us that have yet to be revealed.
Ryota
CC BY-SA 3.0
2017-05-26T05:00:50.787Z
Yup, more complete now. I'd forgotten that this was before he realized how he controlled the titans. +1
Arcane
CC BY-SA 3.0
2017-05-25T08:25:37.780Z
2017-05-25T12:00:08.610Z
2017-05-25T12:00:08.610Z
lentinant
<p>I'm not sure in which chapter this took place exactly but </p>

<blockquote class="spoiler">
  <p> this is after Rod Reiss transformed into a titan. Eren has the ability control titans, then why wasn't Eren able to control him with his titan power? He had Historia by his side who possesses royal blood. That doesn't make sense...</p>
</blockquote>

<p>Why couldn't Rod Reiss be controlled?</p>

CC BY-SA 3.0
2017-05-24T09:19:06.567Z
Made some changes to make the question description spoiler free and keep away the people who haven't read the manga. Feel free to revert if I made the question different from what you wanted to convey.
Arcane
<attack-on-titan>
Manga Spoilers: Control over Rod Reiss
# Manga Spoilers: Control over Rod Reiss
Posted by: username_0 on 2017-05-23
I'm not sure in which chapter this took place exactly but

> this is after Rod Reiss transformed into a titan. Eren has the ability control titans, then why wasn't Eren able to control him with his titan power? He had Historia by his side who possesses royal blood. That doesn't make sense...

Why couldn't Rod Reiss be controlled?
**Tags:** attack-on-titan
> **Comments:**
> **username_1**: Made some changes to make the question description spoiler free and keep away the people who haven't read the manga. Feel free to revert if I made the question different from what you wanted to convey.  
---
## 2 answers
### Answer by username_1 on 2017-05-24. Score: 2 | Accepted answer
A very short answer is **because Eren doesn't know how to.**

Spoilers below:

> Eren has what that has been called as **"The Co-ordinate"** which was passed to him from Freida Reiss through his father. This is the ability of the Founding Titan which allowed them control over the titans and memory manipulation.  
>  We do not yet know how exactly this power works. The real limit of this power is unknown, but as it can be observed, it extends to the point of making Titans perform actions that threaten their own lives. It was also mentioned by Rod Reiss that the power is so strong, it has the potential to wipe out all the Titans if the user is able to wield it to its full strength.

Then why it was Eren was unable to use it while he had contact with a royal bloodline, the reason is that Eren has only shown this power *under very rare and unique circumstances*. One can argue that the danger to his life was one such circumstance, but unless the writer clarfies the exact nature of this power we don't know the limitations.

For more information, with **HEAVY SPOILERS** refer to : [Wikia : Founding Titan](http://attackontitan.wikia.com/wiki/Founding_Titan)
> **Comments:**
> **username_0** (2017-05-24): Thank you! The detail and coherence of your answer is just magnificient.  
---
### Answer by username_2 on 2017-05-25. Score: 2
Heavy spoilers under spoiler section (until manga chapter 89 including)

> Eren is able to use Founding Titan power only when in direct physical contact with person of royal blood. He comes to this conclusion in chapter 89, which also explains events of chapter 50 (titans got Eren's order, while he had direct contact with Dina Fritz's titan), and chapter 62 (when Eren was able to see memories, stored in his titan, while Historia was touching him).

Now, regarding your question:

> Actual combat with Rod Reiss in his titan form has started somewhere around chapter 68, which was a lot earlier. So, basically, on that time, he had no idea, how to activate this power.
> **Comments:**
> **username_1** (2017-05-25): The OP i believe understands the restriction of the Royal Family, but is asking about a specific use of the Founding Titan's power even when he had Historia near him.  
> **username_2** (2017-05-25): @Arcane well, I've missed the part, where OP mentions, that he knows that royal blood is required. Yet, I've referenced chapter, where Eren actually came to this conclusion. Which is far later than chapters where Eren and Co were fighting Rod. But, I suppose, I will update answer with this info.  
> **username_0** (2017-05-25): First of all, I thought that it is enough for a Royal Family member to be around Eren in order for him to use the "Titan Control" power. But we can deduce that he actually has to be touched by them. That was absolutely not the case during the battle against Rod Reiss' titan. However, shortly after unintentionally commanding titans to kill Dina, he commanded them to eat Reiner. Thus, we can deduce that his power lasts for a specific amount of time after touching them. But these are only theories. Just as Arcane said, Eren's powers are still a mistery to us that have yet to be revealed.  
> **username_1** (2017-05-26): Yup, more complete now. I'd forgotten that this was before he realized how he controlled the titans. +1  
---
