Use of たら in この権益の擁護には真剣たらざるを得ない


たら is a grammar structure which means if...then.  
But what does たら mean in this sentence?

> この権益の擁護には真剣たらざるを得ない

The sentence means

> We must be serious about defending this interest

What is the role of たら in this sentence?  
It seems that it does not mean if...then here.




What you see is not the conditional -たら, but
-[たる](https://dictionary.goo.ne.jp/jn/139292/meaning/m0u/) +
[ざるを得ない](https://dictionary.goo.ne.jp/jn/89866/meaning/m0u/).

  * X **たる** ("be X; being X")  
This form is a vestige of an old sibling of what is now called _na-adjective_
, which still occasionally appears in formal speech. It stands particularly
for state that is _acquired_ (instead of _inherent_ ), _temporary_ (↔
_permanent_ ), _apparent_ (↔ _internal_ ), or _due_ (↔ _actually is_ ).

Despite what some dictionaries say, it _does_ conjugate in the same manner as
verbs なる or おわる (consonant-stem; type I) when needed, though most frequently
seen in the dictionary form as an attributive to a noun.

  * **ざるを得ない** ("have no choice but")  
A fixed phrase to learn by rote, but consists of ざる ( _archaic_ "not -ing") +
を (accusative) + 得る ("be able") + ない ("not"). Since it starts with a negative,
words that come before it must be changed into negative form:

> **行か** ざるを得ない  
>  **見** ざるを得ない  
>  **同意せ** ざるを得ない (⚠ _suru verbs have an irregular form!_ )

and

> **真剣たら** ざるを得ない _cannot but assume a serious attitude_




What you see is not the conditional -たら, but
-[たる](https://dictionary.goo.ne.jp/jn/139292/meaning/m0u/) +
[ざるを得ない](https://dictionary.goo.ne.jp/jn/89866/meaning/m0u/).

  * X **たる** ("be X; being X")  
This form is a vestige of an old sibling of what is now called _na-adjective_
, which still occasionally appears in formal speech. It stands particularly
for state that is _acquired_ (instead of _inherent_ ), _temporary_ (↔
_permanent_ ), _apparent_ (↔ _internal_ ), or _due_ (↔ _actually is_ ).

Despite what some dictionaries say, it _does_ conjugate in the same manner as
verbs なる or おわる (consonant-stem; type I) when needed, though most frequently
seen in the dictionary form as an attributive to a noun.

  * **ざるを得ない** ("have no choice but")  
A fixed phrase to learn by rote, but consists of ざる ( _archaic_ "not -ing") +
を (accusative) + 得る ("be able") + ない ("not"). Since it starts with a negative,
words that come before it must be changed into negative form:

> **行か** ざるを得ない  
>  **見** ざるを得ない  
>  **同意せ** ざるを得ない (⚠ _suru verbs have an irregular form!_ )

and

> **真剣たら** ざるを得ない _cannot but assume a serious attitude_

